So it may very well wind up being 'just' setting a flag to false, starting a selective deletion and informing the user that the deletion may take several <units of time>.

It isn't a different platform, specifically because they didn't want to build up yet another social graph from scratch. Your graph in the Instagram app is your graph in the Threads app. This is somewhat akin to how Messenger isn't a different platform from the rest of Facebook.

There may be a future 'downgrade' function, but removing those orphaned threads posts amounts to a partial deletion of your Instagram account. The text that Meta is investigating this is actually in the slide show, but I don't know if they have said anything about what level of commitment they have given, or timeframe they are shooting for.