Technically, Threads uses ActivityPub and is federated with other Mastodon instances (and technically the rest of the fediverse that haven't federated from Meta).

https://blog.joinmastodon.org/2023/07/what-to-know-about-thr...

I suppose the ads are mostly displayed through the Threads app and web server? Otherwise I don't see the Fediverse taking it kindly with being polluted with ads from Meta.