Earlier quoted context omitted.
I think you're right that all three parties have been subject to a certain degree of drift. However, applying that back to the original point (is the current regime the same one that did Tiananmen), to me that's an argument that, for the most part , this is the same regime. And if the original commenter wasn't arguing about a continuity in ideology or personnel, then I'm not sure what they were arguing.
It's like saying Hollande and Chirac are both part of the 5th Republic regime. Technically true but not indicative of anything and probably a red herring.
The views of the same party on a narrow subject in the same country are more similar to each other than can be captured by analogy to the full spectrum of ideology of a western first world democracy.