Balloons like that float very slowly and from the brief description given by China, it uses some sort of steering which is almost surely very slow.
They couldn't have sent it in a short amount of time. Its also implausible that China could predict that civilians would see it while flying in a plane.
Instead, it's likely a routine movement of an aerial platform to keep tabs on the Americans, which was discovered by accident.
It sits so far up that its not entirely trivial to shoot it down. A thaad or sm6 should not have the correct target profile to hit something like that. Getting a gun so far up into the air could be difficult.
But balloon should be going nowhere. So US has some time to decide how to bring it down. These are almost certainly 1 time use disposable items.
It is a rude shock to USA. Otherwise they wouldn't react on a government level scale.
