We build this entire reddit alternative without interconnectivity with reddit, why couldn’t we do that again if threads decides to do that.

We could, but we’d basically have to start over again. It wouldn’t be quite from scratch, but it’d be pretty close.

Plus the format of lemmy is completely different from threads right?

It depends on how Facebook implements ActivityPub. For comparison, Mastodon and Lemmy both use ActivityPub. Mastodon users can actually search for and comment on Lemmy posts (each Lemmy post and comment appears as a new Mastodon post), but, due to Mastodon having a specific option in ActivityPub turned off (I don’t remember which one), the reverse is not true.