"Tankies" are not communists, they are fascists with red masks on

If we're going to get academic about it, the political compass is an imprecise tool and it's a fool's errand to take an absolutist approach to assigning political ideologies on the spectrum. :P Just because an ideology is generally in the authoritarian-socialist quadrant doesn't mean it can't crib notes from a philosophy in a different quadrant. The authoritarian axis is more anchoring here than socialist/conservative.

This is like a small child's understanding of politics

I mean if you want to get academic about it - I suggest you talk to some people in academia. Nearly all of those who say "Fascism is different." self-describe themselves as Marxists or are Stalinist apologists - such as Michael Parenti or Sheila Fitzpatrick. Gee, I wonder why Marxists would want to distance Communism (and more specifically - Stalinism) from being compared with other fascists regimes like the Nazis. Maybe they don't like how it makes them look bad?

https://en.wikipedia.org/wiki/Comparison_of_Nazism_and_Stalinism

No really - read the above link and anyone who is cited as saying Stalinism isn't fascism check their wiki profile. Without a doubt they're a self-described Marxist. They're quite literally the only people who deny that Stalinism was a form of fascism and play apologists for all the atrocities that took place.

A disturbing amount of Communists see attacks on Stalinism as an attack on Communism and so make all sorts of mental gymnastics to defend Stalinism.

Edit:

I much prefer the "That wasn't real Communism!" brand of Communists over the "That was real Communism and here's why the mass murder they did was justified or never happened (despite being well-documented by the Stalinists/Maoists themselves)!" brand of Communists.