(I believe the joke was pointing to liberals supporting a fascist genocide vis-à-vis netanyahu, which is an accusation typically directed at 'tankies')
The general usage refers to people who are so brainwashed that they will ardently deny and even defend massive crimes against humanity committed by their government. In the case of China, deniers of Tienanmen or the Uyghur Genocide would apply. In the case of America anyone that denies israel is committing Genocide applies.
And the glorious leader supporting that Genocide is none other than Genocide Joe Biden.
Calling someone a tankie is saying "You're a piece of shit not only because you support these atrocities, but you dare call yourself a communist while doing so."
Except Trump is the one who wants to "finish the problem", referring to Palestinians. He is also the one, who moved the US embassy to Jerusalem during his presidency, and he's a good friend of Netanyahu the fascist. That's why Netanyahu clearly favors Trump and his fascist Republicans.
Not sure about the Biden part, but otherwise that tracks quite well actually. Tankies do like to make apologies for Xi and the CCP and deny the Uighur Genocide.
This strongly suggests that Zionists are Tankies in fact, which I don't think either group would like to recognize.
I don't think any of the tankies I've seen on this site have been Zionists – they usually use Israel as an example of Western imperialism actually. I'd find it kind of weird for them to be Zionists, since that part of the Middle East has a LOT of socialism – the PLO is made up of mostly socialist & social-democratic entities (led by the left-leaning Fatah party), and is the "legitimate" Palestinian West Bank government, although they're the opposition coalition in the territory's legislature, which is dominated by Hamas since the country's last election in 2006 (the next election was supposed to happen in 2021, but it was and still is postponed by Hamas due to Israel continuously blocking Jerusalem's Arab residents from voting; the recent genocide has seen a sharp drop in Fatah's popularity since they primarily advocated for peaceful means of resolving the conflict).
Tankies ARE definitely genocide deniers in another sense though (Uyghurs) so it's not much better. I remember a lemmy.ml comment I posted getting me banned from the site, since it was maps of countries' foreign investment amounts from China, countries' economic reliances on China, and countries that voted against recognizing the Uyghur genocide (a China-enjoying commenter was arguing that predominantly muslim countries' governments didn't vote for condemning the Uyghur genocide in the UN, therefore it's unreasonable to say that there is one; of course, the governments in question were almost all relatively poor/impoverished/struggling/corrupt countries heavily funded by China that also relied a lot upon trade with China, which would suffer greatly from making diplomatic moves against the CCP – it also aligns relatively well with countries that don't have diplomatic relations with Taiwan, of course).
Yeah I haven't actually spotted Zionist tankies ever.
Though it would make sense for socialists to really hate Hamas given that it's a fascist organization that massacred people living in Kibbutz's which are socialist communes.
But tankies don't really care for socialism, I don't think most of them even know what it is. Tankies are generally simple minded people that like the aesthetic of Che Guevara t-shirts, and red flags with yellow symbols on them. It's about being rebellious against whatever they feel mommy and daddy like. They're generally in university which means mommy and daddy are well off and so socialism feels like something they wouldn't approve of, so they throw on a Che Guevara t-shirt and wave around whichever red flag they think looks the coolest.
So the US is always the bad guy, any allies of of the US are also bad guys, and therefore any adversary of the US or it's allies must be the good guys. Doesn't matter how horrible an enemy of the US is, they will look the other way so they continue to subscribe to this simplistic fantasy world view. At least until they finish university and become the asshole bosses of the working class.