"Tankie" is a word that originates from British communist circles in the 50s. It was used by communists who opposed the Soviet invasion of Hungary to derisively describe their comrades who supported the invasion. Photos in the British newspapers and such showed a lot of imagery Soviet tanks rolling through Budapest, hence "tankie".

So, it has its origins as a silly sounding, incredibly niche, bit of British slang. It faded into obscurity after this, but got picked up again by leftists on the internet as a way of poking fun at each other. It's a funny sounding word and all. At a certain point over the last decade or so, it's breached containment, and liberals, especially here on Lemmy, love to throw it around without really having a firm idea what they mean by it. I've seen everyone from Marxist-Leninists, to Anarchists, to liberal Pro-palestian activists, all called "Tankies".

OP is using the term here to poke fun at that vague liberal usage. As many liberals, at least here on Lemmy, would look at the calculus of, "US foreign policy is more erratic and dangerous than that of China" and declare it a "tankie" talking point, or "simping for authoritarianism", as though making an observation about different nation's foreign policy approaches says anything else about ones opinions of those states other policies or actions.