I think it's a lot simpler than most people here believe: people in Lemmy love the West and they see tankies as people who hate the West. It's nothing more complicated than that. They do not hate fascists in the same way because fascists do not hate the West. Not even Hindutva freaks hate the West if we go by how much they suck off white nationalists on ecks dot com.
Notice how they would lump some Chinese or Russian ultranationalist as tankies even though neither would be a communist, a socialist, or even a progressive. On the flip, you would never see them call a Hindutvist or Turkish Grey Wolves supporter tankies even though they're about as reactionary as those Chinese/Russian ultranationalists.
If you gather a Marxist-Leninist, a Han supremacist, a Russian ultranationalist, a Hindutvist, and a Turkish Grey Wolves supporter all in one room, people in Lemmy would lump the Marxist-Leninist, the Han supremacist, and the Russian ultranationalist as tankies. The one commonality between the three is that they all hate the West.
This has even extended to the Axis of Resistance. They haven't called Hamas or Hezbollah tankies yet, but notice how they use "Assadist" in the same way they would use "tankie." You see them still crying about Assadists even though Assad is just living his life in Moscow.