My own hypothesis is that it's not as much about EEE in the way that people understand it, or about stealing the small amount of users on the fediverse, but more about hedging against the possibility that the fediverse gains significant mainstream appeal. By having one foot in the fediverse they can better capture the fedi-curious. I don't think the fediverse is currently a threat, but the possibility that it could be I think is what they care about. And spinning up Threads is a cheap (for them) way to address that.