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.
The ballon in this youtube video is about the size of a (very) small car, and likely reached the size of a large car as it gained altitude (same gas in ballon, but less air pressure around it equals larger ballon). Granted, that ballon had not just a gopro but also a tray of garlic bread, and went higher up. But using a ballon the size of a small car to launch just a gopro seems incredibly reasonable.
For posterity, I'm pretty confident Anamanguchi were the first men to send Italian food to space, for the Endless Fantasy music video: https://www.youtube.com/watch?v=6fdr-Fiv92c
After it fell back to earth, the local police in New Jersey were nonplussed and thought it might have been a terrorist device. The interview with Anamanaguchi on the topic is well done, but didn't surface after some quick DuckDuckGoing.
I assumed they were talking about the payload. FWIW the Chinese balloon's "van size" was specifically the payload size.
I think the vast majority of people on this site understand that balloons get "bigger" when rising in altitude. (quotes because the balloon is the same size but looks deflated closer to the ground)
