I think he's talking about actual gut crippling fear people experience seeing Chaos.
[PHILOSOFAGGOTRY, TEH SECOND REVELATION]
And from what I've seen, negative human emotions like fear, sadness and misery sort of equally nourish all daemons. Fear seems applicable to Nurgle since so many turn to him due to the fear of death, but that's not quite the same thing as "fear is my aspect." It wouldn't make a lot of sense to have a Chaos God of, essentially, water and or bread, because Chaos in general gets off on scaring people.
The reason no one really claims fear is that you can't do anything with it, as the person experiencing it. Rage, lust, scheming, deathless goo-i-ness, these are all things the "blessed" can use to further the work of the Chaos Gods or their own twisted agendas, as emotions they can both experience and inflict.
I mean, what good is a Chaos Champion that stands around knees quaking and shitting themselves as an act of worship? Or better still, what good is fear as an emotion if they experience it but then master it to do what they need to do (talk to a daemon, fight a space marine, etc....) Khornite Champions and Daemons don't win fights by going "Man I'm too angry, need to focus." Slaaneshi don't win by going "Fuck man this is serious, I need to sober up!" Tzeentchians don't win by going "Keep it simple, stupid." Nurglings don't win by going "I need to be presentable for this fight." (Ironically in weakly written 40k, this tends to actually happen. I hate it when Bloodthirsters suddenly get savvy and cunning and witty, or Tzeentch Daemons suddenly opt for a struggle of arms or Slaaneshi Daemons suddenly become as competent and sober as the story requires. Nurgle Daemons generally are the only ones that stay true to form. The fucking HH novels are rife with this kind of writing. Goddamn you James Swallow.)
In essence fear is just too simple, too basic, too important to the setting of Chaos to weave into a full god portfolio. It's the minor torment that no day is complete without, but in the end, the Chaos Gods all have bigger fish to fry. Really I think my point is: there's no ascension in the ways of Chaos through fear. No one in my memory has seized the power of Chaos with both hands because they're deathly afraid. They might DO something out of fear, but the emotion is quickly replaced by whatever the outcome of their behavior was. Get scared, murder a guy you think is going to murder you? You're in Khorne's Alley and bloodlust is the name of the game. Get scared and submit yourself to Nurgle? Now you're living the Unlife Fantastique and reveling in being unable to die. Get scared and eat a whole bunch of drugs to counteract it? Now you're up in the Hedonism and Drug Abuse Fun House and realizing you can beat fear by just experiencing something else. Get scared and call out to something to help you? Well, now you're in the Daemon Summoning Road Show, and there's nothing more destructive to fear than being empowered. Fear is a gateway drug perhaps to the wider Realm of Chaos.
I suppose if GWS had gone for a Chaos God for every day of the week, Fear would have its due. (Also I seem to recall that Malal may have claimed it? Or maybe that was Terror.) But I think it's smarter to have fewer gods with broader aspects. It's the devotees of Chaos and what they do with the philosophies they align themselves with that makes for interesting reading to me. (And why the Word Bearer's brand of Chaos is so mind-numbingly bland and repetitive. Because it's just "evil" and "Chaos-y".) Fear is just too easy to say it should be valid because you immediately go "Oh I terrorize people." But a good aspect IMO has to have applications outside of just making beings in the material universe miserable. Each emotion has to be both weapon and armor for its devotees. It has to have different applications depending on whether it's being given or received. It has to be deplorable and desirable on same level, both at the same time. And I don't think many people desire to live in fear, in pretty much any incarnation. They may take actions and hold beliefs that cause them to live in fear, but they don't do it because fear is their end goal for themselves. And if there ARE such people with a weird fear fetish, chances are they're numbers are so small at best they're represented by some mid-level Daemon in the Warp, not a full blown Chaos god.
And, if I'm honest, I've always felt that a Chaos God of Fear is the laziest form of Edge Lord Bullshit in 40k. Everything is already covered in spikes and skulls and screaming faces wailing the Song of the Damned. Half of everything is already night black and shadows. Your garden variety daemon already fits that profile. How in the hell do you make something even scarier than that without it becoming a parody of the concept? "Oh I stop people's hearts I'm so scary." Already have daemons that do that as a natural function of daemonry. "Oh I engulf whole planets in crippling terror until civil disorder and Chaos worship sets in." Shit, humans can do that without Chaos' help if they really want to. "Oh I'm like formless darkness and smoke and shit." I.e., the cantrip a lot of daemons open up with before they show their true form. It's just hard to claim Fear as iconic in a universe that is already this scary.
*flamer nozzles light and promethium starts rushing from tanks*
On the Chaos side of 40k, the genre is practically survival horror. The Chaos Gods have to step a level above that to be interesting and distinct in the setting. Khorne is visceral physical horror. Nurgle is straight up body horror. Tzeentch is scope horror (when the human mind perceives things far larger than them and breaks.) And Slaanesh is morality horror. Put another way, Chaos is a slasher film combined with a disease film, combined with a Lovecraft film, combined with a Dark Smut movie. What's common to all four of those movies? Fear for the protagonists. What kind of movie would you add to that line up that didn't feel redundant and dealt specifically with Fear? The Blair Witch Project? Every Horror Movie ever made where something stalks the main character throughout the movie?
Because if all you're doing is just scaring people real good before you kill them, then that makes you a bog-standard daemon in my book, and Daemons don't really experience fear except as a novelty.