We all need to remember online spaces like reddit generally lean younger and more liberal. We never really get a holistic view of any situation. Just as people on reddit would say “we didn’t want trump” and the response was “clearly over half of you did” from europeans, this is another example of how we have to realize we are in our own little bubble in these online communities.
It essentially is calling someone a communist supporter. In 1956 Hungry had an uprising against the soviet union (USSR) communist rule. The soviets used tanks to crush the revolutnaries swiftly and used a similar tactic in 1968 to quell similar events in Czechia. When members of the communist party of Great Britain showed support for these actions it became common in GB to refer to them as tankies referencing the attack on hungry. The term has now expanded its use from exclusive to brittish political discourse to more global/universal label for a sympathizer to opressive communist regimes.