If your instance starts to try to “convert” people off of Threads, they can (and will) just block you.
And then responded to it by saying:
…why would they do that?
That is literally asking why they would block instances trying to convert users into fediverse users instead of Threads users.
Do you work with Meta?
Do you?
me: Data portability is being solved with export standards, so that users can (more) easily migrate to other services.
you: Are you not aware that WhatsApp is also interoperating to comply with DMA? Another Meta company?
I think you are conflating portability with interoperability. Those are 2 separate requirements.
Portability is about preventing platform lock-in, making it so that users can leave a platform (i.e. Threads), and take their data with them to another platform (any platform, not just ones of the originator's choosing). This is not solved with federation.
Interoperability is the ability for users of one platform to interact with users of another platform, without platform-imposed loss of functionality. Whether ActivityPub can serve as a replacement for an API is something that courts in the EU would have to decide. It is certainly not 1:1.