"Really existing socialism" of course meaning "a system wherein workers have zero effective political power."

Just because it's true that the ML movement was an essential part of decolonialization, and because it isn't true that the USSR was some evil empire, doesn't mean that the ideologies that underpin(ned) those societies aren't deeply flawed.

The USSR was not an evil empire, no, but the political structure of a hierarchical, command-based politic lead exactly where critics said it would lead. The "ultraleft", as you call them, including Luxemburg and Anarchist communists warned Lenin exactly what would go wrong in the USSR, and Lenin did not listen.

That's why Lenin is a counter-revolutionary by deed if not by intent. By his actions, the power of the people's and worker's soviets were shattered and replaced with corrupt bureaucracy.

Also, hey, go tell a Tartar, Kalmykk or other displaced ethnic group who were victim of Stalin's genocide that he wasn't a deranged butcher. Maybe if you survive you can tell me what you learned.

Tankies [1] don’t usually believe that Stalin or Mao “did nothing wrong,” although many do use that phrase for effect (this is the internet, remember)

Fair enough. Though I do disagree that they don't usually deny their shortcomings but both sides of that claim are pretty hard to prove so I concede.

We believe that Stalin and Mao were committed socialists who, despite their mistakes, did much more for humanity than most of the bourgeois politicians who are typically put forward as role models (Washington? Jefferson? JFK? Jimmy Carter?), and that they haven’t been judged according to the same standard as those bourgeois politicians.

Ok you can make that argument

People call this “whataboutism”

"Tankies" do often use whataboutism, that's irrefutable. Is this specific claim whataboutism? I'd say borderline, but I can see why it's still a point worth bringing up.

but the claim “Stalin was a monster” is implicitly a comparative claim

No, it's not - or at least not in the way he's implying. The claim is overly vague (how do we define a monster?) but it's not comparative. Whether Churchill committed atrocities (he did) and whether Stalin committed atrocities (he did) have no bearing on one another. All we have to do is define a monster - then we can measure whether a given leader was a monster. The only comparison needed is between the leader and the definition.

If people are going to make veiled comparisons, us tankies have the right to answer with open ones.

Sure, that's true. Except like I said that "Stalin was a monster" is not comparative. If someone says "Stalin was worse than Churchill" than Churchill is relevant. But if someone says "Stalin committed atrocities" then it is whataboutism to answer "So did Churchill." Churchill's atrocities bear no relevance to Stalin's.

A tankie is a leftist who doesn’t agree with mainstream geopolitical opinions

This is a stretch. Leftism, by and large, doesn't agree with "mainstream geopolitical opinions" so this doesn't properly distinguish them from other Leftists.

A tankie is a leftist who... shows any interest in nuance

Laughable. Tankies originate from Leftists walking the party line so claiming that all non-tankies lack "nuance" is a very... interesting accusation.