The fact that Threads has a much larger userbase than Mastodon already means that they won't ever really feel threatened by it. The stated reasons why GChat and Facebook Messenger eventually defederated is that it was hard to keep scaling the platform while speaking XMPP, but the unstated reasons were that Messenger and GChat at the time were still very much niche technologies that were jockeying for marketshare in a crowded space.
This time around Threads is already an order-of-magnitude larger than the existing Mastodon Fediverse. Moreover, now Meta has a diverse array of different social products, so there isn't as much pressure on any one product to succeed. If Threads ends up in a dominant position in the threaded-text social network world, that already nets them more users and more opportunities for ad revenue, which they can collect revenue aside their existing properties of Facebook, Instagram, and WhatsApp. On the other hand, interoperating with the Fediverse allows them to be opinionated about what kind of content they allow on their network (e.g. if you're posting from Threads, you can't post sexually explicit content) which can keep them advertiser friendly, while offering a relief valve for the loud minority that will want content disallowed by Meta's content policies. It's a win-win really.
This time around Threads is already an order-of-magnitude larger than the existing Mastodon Fediverse. Moreover, now Meta has a diverse array of different social products, so there isn't as much pressure on any one product to succeed. If Threads ends up in a dominant position in the threaded-text social network world, that already nets them more users and more opportunities for ad revenue, which they can collect revenue aside their existing properties of Facebook, Instagram, and WhatsApp. On the other hand, interoperating with the Fediverse allows them to be opinionated about what kind of content they allow on their network (e.g. if you're posting from Threads, you can't post sexually explicit content) which can keep them advertiser friendly, while offering a relief valve for the loud minority that will want content disallowed by Meta's content policies. It's a win-win really.