They flew a F35 next to the object. "Unknown" is an overloaded term and it isn't binary (known vs unknown). Plus, they said it was the size of a small car (remember the other balloon's payload was the size of a bus). So while you don't know what exactly that payload is and is doing, you do know it isn't just a go pro. You also know it isn't a velociraptor or Felix Baumgartner.