It’s different because for a while, there was no incentive for the average person to move to decentralised, libre XMPP when they could interact with the people there from Google Talk. If someone was on XMPP and asked a Talk user to come over, the odds they would be convinced were very small. Why would they when they can access it anyways?
Even if they didn’t kill it, they certainly stalled it’s growth, from the moment they federated.
Facebook will probably start not playing nice slowly, if they don’t right away, so it’ll be very hard to draw a line where they should be defederated, so larger instances and users may never end up defederating them thanks to blurred lines and constantly shifting goalposts. “We’ll defederate when they don’t play nice” is too vague to work when they move slowly and change things gradually.
When the vicious predator bares it’s fangs, it’s best to deal with it right away by running or fighting back, instead of trying to be friendly with it.