I see it thrown around on lemmy a LOT, just like how woke gets thrown around by the right, I’m not saying it doesn’t have an actual “true” meaning, just that (like most labels as I mentioned and particularly those that are more specific/niche initially) it eventually gets bastardized and thrown around with the original specific meaning lost.
That’s because there are a LOT of tankies on lemmy, specifically lemmy.ml. In many threads anything slightly critical of Russia or China will get deleted and the users banned. It may have other meanings but that’s what it means here: people carrying water for authoritarian regimes while LARPing as leftists.