IRC is actually somewhat federated, as one network can consist of multiple servers hosted by different people.

Short answer: Yes, this is stupid.

I assume if threads users follow fediverse users the content makes its way to their feed as normal.

No. https://woof.group/@aphyr/113596102626961803

You can now follow fediverse accounts on Threads, but the accounts aren’t searchable and their posts won’t show in feeds

what’s the point then

They can be URNs (like magnet:), as shown above.

It’s not true, all ActivityPub IDs are URIs

ActivityPub spec allows content-addressed IDs, just nobody implemented them and now it’s too late.

https://gitlab.com/spritely/golem/blob/master/README.org