"Embrace, Extend, Extinguish" is when corporate pigs try to get ahead on open standards, make their client the main one used, then make their client drop the standard.
Email is at risk since most normies use Gmail or Outlook. Google and Microsoft could agree to drop support for all other providers and people probably won't switch to the other providers.
The most Meta can do is add a load of more users to the fediverse then take them away again. For them to successfully EEE the fediverse, it would require convincing existing fediverse users to switch to threads. I cannot see that happening on here on any noticeable scale.
The worst that can happen, is a bunch of new users appear on the 'verse from threads, then disappear again.
If anything, Bluesky is the bigger threat as it touts itself as "decentralised" in order to gain users who would have otherwise gone to Mastodon, then easily pull the plug on that, and we have to wait another decade for a maniac to ruin the platform to cause people to reconsider the fediverse.
I'm down with the notion of resilience in fedi, but I don't know if that's necessarily the direct worry.
In the immediate, more trouble could be made at the protocol level, which I think is where EEE is more relevant, i.e. specific technologies rather than federation as a whole.
So in my mind, even bluesky might not have as much potential to mettle with, for example, activitypub. That is unless they can usurp it and make atproto what everyone is using, or pivot to it and take it over directly. They have no real sense of control, except in their bubble. Drowning out the grassroots fedi might be a real concern, but I'm not sure if that's exactly EEE or not. Rather it would keep grassroots small while growing the non-grassroots presence, but not necessarily destroy it. That might just be semantics though.
I guess my point boils down to the fedi "network" vs the "plumbing" of the protocol. One is a concept, the other is concrete tech. And I agree there isn't as much concrete threat as perceived, anyway.