It’s not stupid, you shouldn’t be able to discover users in one service from another unless those user have explicitly opted in. It’s not a technical issue, it’s a principals issue.
In this case, you create a Threads account and you should be able to discover everyone else who has also made a Threads account (cos that’s the point), but you shouldn’t be able to see Instagram only users unless they have explicitly said that’s ok.
This is probably all ‘explained’ in the T&C’s, buts that’s getting into a whole other thing