Those aren't the same propositions (I'm not sure they even are propositions?) so there is no such logical relationship.
"Meta will nuke your account" and "you nuke your account" is different. It might not even be possible to perform one of these (nuking your thread profile).
Your Instagram account is deleted (the state not the action) iff your thread profile is deleted.
Your thread profile is deleted iff your Instagram is deleted.
Those are the propositions that are logically equivalent.
One says that if your intent is to delete your Threads profile the only way to do so is to delete your Instagram account. The second implies (unsupported) that there's a way to delete your Threads profile that a user might not understand will also delete your Instagram account.
The distinction is around intent. The parent's phrasing makes it seem as if its done for technical reasons (e.g. your threads profile and Instagram profile are actually the same thing) while the headline makes it seem like a business choice.
Read them back carefully - they're two different sides of a coin. One is talking about deleting your Threads account, the other is about deleting your Instagram account.
Meta will delete Threads profile only if you nuke your Instagram account
and
Meta will nuke your Instagram account if you delete Threads profile
mean exactly the same. In fact they are logically the same. It's like A iff B, means the same as B iff A.