If bluesky ends up getting implemented by multiple vendors, then the IETF might make sense. But its a bit early for that at this point.
I suspect the bluesky team might also be able to make big changes to the protocol in private for the time being, so they can iterate rapidly. Iterating on a protocol at the IETF would be way slower. The IETF usually takes years to publish big, complex protocols like this. Look how long it took http3 to work its way through the pipeline.
https://atproto.com/specs/atp#future-work
> Protocol Governance and Formal Standards Process: The current development focus is to demonstrate all the core protocol features via the reference implementation, including open federation. After that milestone, the intent is to stabilize the lower-level protocol and submit the specification for independent review and revision through a standards body such as the IETF or the W3C.