Trump did not have the largest inauguration crowd in history, it’s easily observed by looking at photos of the event compared to other inaugurations

A "fact checking service" that had a deliberate bias could easily work around that. They could claim that the pictures chosen are not representative. They could talk about how crowded people were in one picture vs. another. They could claim that certain people seen in the crowd were not actually there for the inauguration, but were protesters or something.

Every fact checker agrees on that fact.

That's because there hasn't been any reason to set up a Russian Fact Checker service. Instead they just question the credibility of the existing fact checkers. But, if there were a reason for a Russian Fact Checker service, there would definitely be one.

If there were a rogue fact checker that regularly went against clearly evident fact

How could you tell? Look at all the people living in the MAGA bubble. They don't understand that everything in that bubble goes against clearly evident facts. But, what could happen to them could happen to anyone if the disinformation was strong enough.