To add to the excellent points here: they are incredibly propagandised and treat-brained. While they recognise the social evils of fascism and some of the micro oppressions of capitalism, any position or person that asks them to admit that current systems are fundamentally, structurally broken (or worse, to do something about it) provokes a wave of anxiety about losing what they know and guilt about benefiting from the system/not coming across as leftist as they wish to appear. “Tankie” is simply a deflection, a manifestation of their avoidant behaviour so that they don’t have to feel those difficult, shameful, but ultimately growth-provoking feelings.

Also, by co-opting a term from leftist infighting, they can signal that they consider themselves lefties, without needing to actually engage in any of the critical thought required.