The problem is that Marx and Hegel denounce voidist looks at society. Both say analyze everything that is related to it and how they are related to each other. (Your reporting of) Bookchin did the equivalent of libel in a philosophical sense when talking about DiaMat and Dialectical Idealism. While Hegel was more inclined with the History of Philosophy and how that related to Absolute Idealism but denounced Metaphysical voidist analysis, and Marx was also Dialectical and Historical Materialist in his analysis in that he denounced one-sided views and analyses of anything.

So while Hegel would not care of nature, because of the Absolute Idea, yet it would be Marx that made the tools for us to be able to analyze nature Dialectically - which (your reporting of) Bookchin is seeming to deny. For while Marx may have some slight pro-human inclinations, he was anti-humanist (denies humancentric views). Regardless, Marx could only stand on the shoulders of his giants at his time and we know more than him now and should make more analyses than he could ever dream of because we stand on his shoulders now. And DiaMat and Historical Materialism has no conflictions with Nature and including it in its analyses, but Bookchin’s facsimile of it does and that’s what worries about even reading him.

Also the End of History is not bad Dialectics but an end to something that can progress no more further. For Hegel, of course it was the realization of the Absolute Idea, but for Marx it was society no longer progressing because all the contradictions between classes, production and, ultimately, people would be resolved. Though Hegel had his words with it, Marx didn’t think of perfect society at this stage, just that contradicitions will be resolved as Communism had done its job to abolish (Dialectically negatively supersede) all contradictions of Bourgeois society. But a denial of an End of History is to ultimately think that contradictions can’t be resolved or they’ll appear for no apparent reason after old contradictions were resolved - which that is bad Dialectics and denies the dynamicism of society and material conditions.

Now to talk of Full Communism is only that, that the End of History has come to human society. Space exploration is a part of Full Communism and the extension of such, there is nothing new to gain but Dialectically positive supersessions of the same stuff in different conditions. Marx and Hegel both talk about Dialectically positive supersessions as they do with Dialectically negative supersessions, just (your reporting of) Bookchin failed to realize that (hence first as tragedy, second as Farce from both Hegel and Marx). So we can have space exploration, but that only reproduces our society under different conditions.

Now, the alien invasion is more of a hypothetical we can’t imagine. Both in Bookchin’s case and Marx’s and Hegel’s case. Bookchin couldn’t predict what could be fathomable from that, but Marx (and maybe Hegel?) would say, like any good materialist, that it doesn’t necessarily change anything but how society reproduces itself on all levels. So if we win, we just Dialectically positively supersede our current conditions with, possibly, better alien tech. If we lose, you and I and nobody can fathom if they’ll disintegrate (lazer) us to death or what. Regardless, it’ll all be farces of what happened earlier in human history. And this is all assuming that all aliens are malicious space imperialists.