As is the argument that racial, class, and other distinctions would not be solved at all, so the idea that it would be an idealized oppression-free society (conceded here, but... here's not the problem - and once all the non-radical people who bought into a separatist movement find they've been sold a bill of goods, it's going to be that much harder for any solution to get implemented.) is invalid.
Last I checked, it's not a proposition for solving all oppression, so much as one particular type. And I don't know that it's been billed as such. So that doesn't strike me as a flaw, so much as a nirvana fallacy: if you're not solving every problem at once, don't solve any of them.
That said, it's a good reason why the whole thing wouldn't interest a lot of women, much like feminism doesn't interest a lot of WOC: it doesn't address their issues to the same extent that it does white women. Which is something of a problem that feminism is wrestling with (or stubbornly and stupidly refusing to wrestle with, sometimes).
Unfortunately, the only way to work that principle into a separatist paradigm would be to have communities further fragment along racial lines. I wonder if that's a good idea.
Since the attitudes of the people who propose it can be generalized directly to how the idea would actually be implemented, every problem with the people who propose it is a problem with the idea.
True enough. And it's one of the places where the implementation by those people would fall down. But that doesn't make the other objections against the idea of lesbian separatism (that the idea itself discriminates against or alienates men, for example) valid. That is, there are objections to the idea at its core, which is "women should separate themselves from the patriarchy so as to provide a viable alternative to the patriarchy and therefore weaken its position", and then there are objections to the people currently promoting the idea. It's kindof like the difference between objecting to socialism and objecting to the french government.