I'm certain nowhere in Marx is the term "post scarcity". If that's what he really meant, then it should be easy to demonstrate that with a paragraph or so. Otherwise one could take any good outcome and insist that Marx already predicted it.
It’s honestly really hard to tell if you’re engaging in good faith here. If you really are, I’d direct you to the Grundrisse. Marx specifically explains that as industrialization continues more and more parts of the economy become automated which (should) free up labor to pursue other things like art and science.
Yes it is true that he didn’t literally use the phrase “post-scarcity” but there has been 200 years of academic development on these works since then and I don’t regard this simpler way of discussing the concept as controversial, especially since this is a social media thread and not an academic paper.