Theoretical aphorisms can't override the facts of recorded history. If the harsh border control and cultural isolation of the East Bloc ran counter to Marxist principles, then you can complain to the dead leaders of the USSR, but that's about it.
Check the history of the different communist movements. Maybe an entrypoint would be this: today there is the Fourth International. The USSR was born out of the Third International with Lenin, and the Fourth International opposed/opposes the USSR's practices and because of that the USSR hunted them down using their agents whom eventually killed Trotski.
Check the history of the different communist movements. Maybe an entrypoint would be this: today there is the Fourth International. The USSR was born out of the Third International with Lenin, and the Fourth International opposed/opposes the USSR's practices and because of that the USSR hunted them down using their agents whom eventually killed Trotski.