Indeed, that is not gatekeeping. It's applying social and moral pressure. Similar to a boycott campaign, protest, etc. Such methods are intended to discomfit and inconvenience. They're used in situations where being nice and getting along are determined to be nonviable strategies for getting the desired result.

Those methods in themselves are morally neutral; the question is, are they employed for a reason which justifies and necessitates them, i.e., how serious is a thread does Facebook pose to the fediverse. (I think it's reasonable to take facebook as an evil seriously and to not give them an inch.)

Some people are very entrenched in their opinion and I wouldn’t be surprised to soon see posts with “We must defederate from everyone who federates with Meta”.

That's definitely already happening. This is a normal part of federation, tbh. Instances block instances that federate with bad actors because they want to limit as much as possible their exposure to/involvement with those actors, as well as to place pressure on others to do the same. Obviously not everyone considers facebook to be a bad actor, but it's not surprising that those who do would act accordingly.