What observational truth? The people in the OP aren't as the user described. The number of people that meet the definition given is close to nil on the Fediverse, yet people get called a tankie all the time.
"Tankie" is just a pejorative for those who uphold existing socialist states as legitimate, which covers the large majority of Marxists and a decent number of anarchists. The people in the OP are "tankies" by the definition I gave, it's consistent and clear. They uphold existing socialist states as legitimate, and this is the norm among Marxists.
Call out whatever you like, it's going to backfire, because what I'm saying is more in line with reality. Unless your point is that Marxism itself is extremist and therefore "tankie," calling the norm position among Marxists extremist isn't particularly useful.