I really appreciate your approach to this, by the way, but I do have a couple of problems with his line of logic, which ultimately boils down to definitions.
The quote "This isn't a slippery slope argument; this is the built-in conclusion to contemporary fascism" sticks out to me as what most reveals this problem to me -- genocide is the built-in conclusion to "contemporary fascism", but it's built-in because he has built this conclusion into his definition of the alt-right from the start.
Fascism is a term with very vague meaning, meaning anything from just being racist to being a literal Nazi to being totalitarian, and in fact I know someone who actually has described himself as fascist, but without a single ounce of racial rhetoric, he literally just wants (maybe wanted) a totalitarian state, sometimes modeled on certain organizational structures of historical fascism, such as national direction of corporations. With the way fascism is used in modern discussion, I suspect most people wouldn't think to label him as fascist at all. Fascism is a very easy label to use as one desires because it can be applied to a wide array of things and then used to portray these things as something much more rigid, specific, and coherent as an ideology.
This is what I believe this guy does. He portrays white nationalism as "contemporary fascism" -- an ideology with a defined goal, which desires it in absolute terms, and which will do whatever it takes in the end to reach that goal, but I don't think that's actually a correct portrayal of the more milquetoast alt-right or even most white nationalists. In general the alt-right holds a loose collection of related reactions and values that they'll get mad about when something happens they don't like, they'll have a pet policy or a pet problem; the alt-right doesn't have a unified logic. Even the concept of "white genocide", which he uses as a core piece of his argument, is something that is kept to a minority of the alt-right, not your Sargons of Akkad, and I'd wager most people who talk about white genocide don't have the full demographic shift conspiracy, but rather a more vague idea of thinking that policies are being made to get rid of them.
His argument holds up with the givens that he's set in order to make it, but I don't think that his givens are actually at all accurate, because they rely on using the definition of fascism's vagueness to its fullest extent.
Frumple:
No, I don't think so. The United States is not a poor country, and it's not a "third world" country even in its poorest areas -- the economic situation is not always fantastic but it is far away from being the point of mass starvation or mass death even in the most miserable parts of the country. This gulf is massive and I think underappreciated, and it would take genuinely cataclysmic events for it to be crossed. Every single one of the major things that can cause that kind of widespread death is out of reach for the United States. There are no areas of the country that are starving to death. There are areas of the country where food insecurity and malnutrition are very prevalent problems, but there are no areas of the country under even the kind of chronic famine you find in impoverished countries. Mass death through violence isn't a problem that exists in the US either -- actual demographic-changing waves of violence and war are magnitudes of order beyond the worst incidents we see in the United States because of this economic base level as well. Disease isn't this kind of issue either; things like heart disease are pretty major problems but actual decimating waves of disease that can meaningfully cause an existential threat to a population aren't possible without some seriously terrifying superbug -- but that kind of superbug has never actually spread on a large level because even superbugs always have alternate methods of treatment, so I'm not seriously worried about that either.
The kind of shock that it would take to upend any of this amounts to more or less destruction of society as a prerequisite. The more I think about it, the more distant the idea of widespread death seems.
As for the one comparison you've made... well... Frumple, frontline soldiers in the middle of a battlefield don't have a high rate of fatality at all. In fact, they have a low rate of fatality. They have an extremely low rate of fatality, it's not a good benchmark. Soldiers in the US don't die at anywhere close to the way people envision them as dying, that's why national news is made every time an American soldier dies abroad. Dying at the rate of frontline soldiers is actually not significant and not very hard to attain.