So if Threads isn’t trying to overwhelm and destroy Mastodon, why have ActivityPub support at all? Two answers. First, “Look, see? We’re open!” is not only perceived as a great talking point these days, it’s perceived as a regulatory relief valve. Look, see? ActivityPub! We’re open!
Second, remember that the business model for Threads is keeping you on Threads. If 95% of your friends are on Threads but 5% are over on that weird Mastodon thing, now you don’t have to use Mastodon to follow them! Just follow them from Threads! Woo! Will Threads be a good Mastodon client? No, but it just has to hit “good enough.”
These are also great points to highlight imo. A throwaway defense against regulation and entrenchment by enabling access to the few friends that outright refuse to use their products both make more sense to me compared to some of the EEE arguments & data collection. I'd be more inclined to see Meta/Facebook doing the bare minimum with ActivityPub and using a strict allow list vs. open federation, but time will tell.