Another thought is that they're not trying to kill Mastodon, they're trying to kill Twitter.
Mastodon has a bit of a community already, so by implementing ActivityPub, Meta can make its platform seem bigger than it is by pulling in Mastodon content. Gives it another edge over Twitter.
Best case scenario is Threads sees ActivityPub as just the cost of doing business. That way, even people who won't use your platform are still interacting with it. Downside, people on your platform can leave for a federated alternative and not miss out on any content. Not sure if that downside makes up for the potential gains.
I think the default approach needs to be defederate first unless Meta shows actual interest in developing the fediverse with good intentions. If Threads become the majority provider of content to the fediverse and then we defederate, we lose all that content. It could lead to Mastodon, Lemmy, and Kbin withering and dying as everyone goes where the content is.