Yes, you are correct!

Knowing the MAC makes the attack reasonable - let's say 5 hours compute for 3080Ti.

Not knowing the MAC makes it exponentially harder. You can still "guess" it, but the search-space is vast and that would take bazillion-years.

So to attack iOS device: - user has to download the app - app has to broadcast fake BLE - some other devices (e.g. Android/RasPi would need to pickup that MAC and pass it to you