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.
When people ask for a definition of tankie I go with "Anyone that supports the soviet union, cuba, vietnam etc." then follow up after others agree with "Like Jeremy Corbyn" to throw a wrench in the machine.