The will federate, try their best to suck as many users from fediverse as possible into threads, then defederate and become a walled garden again.

As for how they will suck users away:

• Make their algorithms prefer posts from threads, so anyone wanting to reach a wider audience needs to move to threads
• Add twitter like checkmarks that are only available for users of threads (they will say it is for security since they need to verify the idwntity of checkmarked people)
• Add features that are not exposed on activity pub, so that you have to be on threads to use them (twitter did the same by for example not making polls available over API)
• Intentionally make their activity pub slow and unreliable to make it look like other instances are broken and threads is fast and reliable.
• Probably much more