You're interpreting the claims more strongly than they were presented, IMO.
> One of its premises is that The Rich are some cohesive group
There doesn't have to be a fully cohesive group of an entire class for there to be negative consequences for the non-members of that class. The members also don't have to be cohesive or aligned at first, but they will tend to align on issues that threaten their position.
For example, we have historically seen that the incumbent elites tend to be anti-socialist/-communist, because their relative position and power are threatened, even if their populations and some (aspiring) elites are remotely pro-socialist. And because the coercive power of incumbents is often much higher than the power of some populace or aspirants (i.e., they have more to offer to those who hold coercive power), they will tend to succeed in pushing against the majority.
The entire history of the US, UK, Germany, France, Russia (etc.) is full of such examples.
> that can trade amongst themselves in a hermetically sealed economy.
It doesn't have to be hermetically sealed to exclude certain populations.
On a global level, many countries don't trade with each other, and their economies are doing fine. Hard sanctions and cold wars even make this intentional, rather than a product of "we don't have anything to offer to each other".
On a local level, most people don't interact with the "homeless"/unhoused, and the latter often don't have much to offer to the former. Most Western countries don't need to hermetically separate the rich from the rest, but if you look at Brazil, South Africa, Saudi Arabia or even the US, gated communities are common in some areas. Most of the rest outside don't have much to offer to those inside.
Taken together: if it's plausible, and we don't know how probable it may be, we may want to figure that out, instead of hoping it's not probable.