Some more details on how Instagram is planning to build an ActivityPub compatible service... https://www.theverge.com/2023/5/19/23730070/instagram-twitter-app-competitor-leak
I don't think they care.
Account portability was such an obvious feature that surely they considered but rejected it as not particularly important to what they intended to build.
ActivityPub is all about instances, not users. It's all centered, and centralized, around instances.
Account portability just doesn't matter to that instance-first organization.