I found reports of similar account hijackings on Reddit. The relevant common factors are that it happens fast, the suspension reason is bogus, and account recovery tools do not work.
To add insult to injury, the message says to log in to insta and follow directions to appeal and the button links to my apparently normal insta account which of course has no instructions on how to appeal anything.
Account hijackings happen, but how does Facebook fail to protect their own account recovery process from being rendered useless? How do they send out account suspended messages linked to an insta account that doesn't have recovery/appeal/whatev steps to take?