The context to this is saying about how they're not a fan of the subreddit LeftWingMaleAdvocates, which is a bad subreddit, it's mostly just anti-feminist and basically just MRA, but they just call themselves left-wing. Most of their "left-wing" stuff is actually just "classical liberal" talking points, so they are bad, but calling them tankie is ridiculous.
100% of the time "Tankie" can be substituted with "Commie" and nothing in the given sentence will change.
You can literally substitute it with Teletubby and nothing will change. It's a thought terminating cliché that is used to prevent any critical thought on subjects outside the status quo.
With how rapidly the status quo is shifting to the right now, it won't be long before Ted Cruz is called a tankie. Everything that isn't rolling tanks into a city to put down protests will be called tankie.