No true Scotsman is an informal logical fallacy, an ad hoc attempt to retain an unreasoned assertion.[1] When faced with a counterexample to a universal claim, rather than denying the counterexample or rejecting the original universal claim, this fallacy modifies the subject of the assertion to exclude the specific case or others like it by rhetoric, without reference to any specific objective rule.
The key part is "without reference to any specific objective rule", which I don't think you're taking into account.
I can't do the same for most other ideologies; there's no Conservative Manifesto as it were.
With that in mind we can take Marx's book and compare it against history in these countries. If you do this then you will come to the conclusion that these regimes were not Marxist and in fact not even close to socialism.
In fact, the Soviets were constantly justifying their policies based on the writings of Marx and Lennon.