I'm not sure what Meta's goal is with adding federation to Threads. Some options include:

• Preempting government scrutiny for monopolistic practices
• Gaining a competitive advantage against Xitter/Bluesky/etc...
• Giving Threads users access to more people/content

As for why I think the flow of users is likely to be away from Threads:

• People who already actively use Mastodon and the like tend to be fairly technically sophisticated and anti-corporate. Not many of those will switch to a Meta product when they can reach the same audience without it.
• Most people who joined Threads got there via Instagram. Those sorts of mainstream users mostly haven't been exposed to decentralized, non-corporate social media. The added exposure of seeing it from Threads is more marketing than these open source projects could ever hope to buy.