I followed the link as you suggested. I found a slight correction on the way it works.

A “shadow account” was some layperson’s attempt to describe what happened. That seemed clear to me immediately. It also seems that Threads and Instagram are much more intertwined than users expect.

I understand why this would upset people! I was furious when I tapped one screen wrong and connected my Facebook and Instagram accounts. It can’t be undone. It changed a profile picture. I didn’t quite become angry enough to delete both, but I stopped using them.