As you’re discovering, Leftism isn’t a universal thing, there are sub-groups ranging from (US) Democrat/ (UK) Labour all the way through to individualistic anarchism. There are a lot of stopping points on the journey, none of which are the same as the others - socialism, communism etc and even within those sub groups there are sub-sub groups.
So the the word you’re questioning ‘tankie’ is a term applied to communists who deny that communist regimes are capable of, or indeed have, committed acts of violence on individuals, groups or entire ethnic groups/populations. The word originates from the Tiananmen Square massacre, where the ruling communist party murdered protesting people using tanks. ‘Tankies’ deny that this happened, or if it did happen it wasn’t as bad as the West claim, or if it was, that it was a justified act. They say the whole thing was made up/exaggerated by Western media. They do this because they’re living in denial.
Most of the Lemmy (and Reddit) based tankies I’ve encountered seem to belong to a youngish American, probably middle-class population. Possibly they’re rebelling against some element of their upbringing or are seeking some kind of group identity or validation. Believing in something that extreme is also an easy thing for someone to do - if you support a political system that is never ever going to assume power where you live, that abdicates you from any responsibility of engaging with the political system in your country to try and make the reality you actually live in any better.