All tankies are communists (purportedly), but not all communists are tankies. Tankies believe in Stalin-style militant communism, and will do or say anything to support their ends regardless of if it's objectively true or not. They will deny all harms that are caused by anyone who claims to be a communist.
Basically they're MAGA in a different claimed direction.
Yup, a few years ago i told a vegetarian that i was trying to eat meat less frequently and they said something like "if you're still eating meat then you aren't doing anything". Seriously? Such a militantly blind all-or-nothing nonsense take. Most people's reaction to that type of tactic is to say fuck it I'm purposely gonna eat even more meat now