Not sure why some people think Tau would be at all weak against Chaos due to not "seeing" it. They see Chaos creatures just fine (it's not like EYE Divine Cybermancy where chaos beings don't show up through cyber-optics). They're just immune to the corruption. They don't get to use psychers, but other than that they're perfect for fighting chaos. I wouldn't be surprised if they got a bonus against certain chaos magicks due to lack of soul and stuff. Though I'm sure summoned flames and daemons hurt them just fine.
The problem is that they're so weak in the soul the lack of power becomes more of a detriment than the lack of notice is a benefit. Tau would not be resistant to magic, they have souls, but those souls are merely small compared to humans and eldar. Resistance is only conveyed by a negative warp signature.
Like, the Tau would have no chance against dealing with a rampaging daemonhost that needs to be banished, because they have no banishers. They can't attack daemons for real without psykers, and they can't use psykers to detect hidden Chaos presence. They've got no chance.
Not resistance per se, no, but harder to notice.
Also, Emperor take you MSH. Seriously.
Nagi.
Nicassar. Hell, fucking Kroot probably have psykers when they eat enough Eldar, Weirdboyz, or even human psykers. They certainly have their own warp travel, which requires psykers of some sort or another. Like, if you wanna make jokes about the Tau having no souls, whatever, but don't just ignore a whole aspect of their style.
It's not just grayskins, ya panzy gits, dey've got plenty uv udder gitz wif em to fite tooAlso, they science
everything. Even if you wanna say they don't yet take Chaos seriously (which I think they do, though maybe not as seriously as they should because they haven't had the Horus Heresy to terrify them into Ecclesiarchy), as soon as the first Daemons hit, it's not just silly folklore about spirits in the night. It's an enemy that can appear at almost any time with very little warning that infects people simply via proximity into becoming slowly brainwashed, with their largest new assimilated species as it's primary host mechanism.
That gets people's attention. They've fought Chaos before, and are doing so now. They have lost worlds to Chaos Warbands, in fact, if I remember right. This isn't a 'DERPADURR' thing anymore. The side effect of GW (or the purpose? I dunno) making them edgier and grimdarker is that they become less naive, which means they get the issue with Chaos. They don't see it as Public Enemy #1 like the Imperium does, because for them, it isn't. They don't have the vulnerabilities of a massive and growing psyker population which has no self-direction or willpower because of a total war economy and indoctrination. I make that second point because the Eldar also have a massive psyker population, they just also happen to have metric shittons of self-direction, to the point of living two full careers, essentially, to maintain discipline and keep themselves occupied and engaged with life to avoid She Who Thirsts.
Tau don't ascribe as much danger to Chaos as they probably should as they start to agglomerate a large human population. They're smart enough to advance to nearly Imperial tech levels, or better (sorta, I get that the Imperium has better weapons, but they certainly don't have the means of producing them in the same relative quantity) in five millenia from pre-sentient life. That should be enough to give you an idea of how quickly they develop stuff.