It’s clear to me some folks don’t know/understand the difference between protocols and services.
#Threads is (allegedly) going to support federation via the #ActivityPub protocol, meaning that users on other services implementing the protocol, like Mastodon, will be able to interact in some form.
That doesn’t mean you’ll be able to use Threads with your Mastodon client. Clients don’t implement the protocol directly, they instead use Mastodon’s API which is specific to Mastodon.