I think you're reading too much into it. "Shitty user-hostile behaviour" would be "Muahahahaha, if anyone ever wants to get rid of their Threads account they must also *delete their Instagram account*, hahahaha, the world is MINE! MINE, I TELL YOU!" <fx: thunder crash, up grams dramatic music>
Is that scenario more or less likely than "fuck it, just use the Insta signup server for auth, we need to ship something *today*" "yeah okay, that'll take me ten minutes"?