It also gives them an edge over Bluesky, since no where else is using the Bluesky protocol yet, whereas ActivityPub has all these sites also using it and populated.
Threads wants to be there place where everyone is happening and everyone feels like they need to be, like Twitter was and Bluesky is starting to be. Mastodon was never that. Mastodon, to them, is a tool to use against Twitter and Bluesky for that pop culture spot, not a rival.