This is clearly an “embrace, extend, extinguish” move.

Is it? I certainly don't think Meta is doing anything because it's genuinely the "right thing", but that doesn't automatically translate to them already having a deliberate plan to kill off the fediverse by starting to use it before they add on their own proprietary features only available to their users.

My personal opinion would be that there's no need to pre-emptively defederate them. Keep yourself ready to defederate the instant they do something that's directly harmful to the community at large, but until then, hold off and let their users experience what the fediverse has to offer—and let our communities benefit from their increased activity.