Wasn't talking about allies, but the domestic systems they operate under. Realpolitik means that the two don't always end up the same - the US has backed dictatorships, democracies, and even the occasional socialist regime, after all. China, likewise, will do the same - back any system as long as they benefit from it. They may generally prefer countries to adopt their system, but their hegemony is not married to that preference.

It also may seem minor, but different authoritarian systems are shit in different ways. ML systems share a great many features - largely due to being imposed wholesale by the Soviets - and their progress (or lack thereof) is like clockwork. Other authoritarian regimes wax and wane in very different ways.

The two countries and their respective predecessors have the longest diplomatic relationship in human history,

I would cast doubts on that.