With modern knowledge of meteorology (both peacetime global monitoring system and national space-based monitoring), there was no 'if' - they could know where and when it'll go. Moreover, it's quite steerable - both globally (chose the necessary stratical layer with the necessary wind direction) and locally (it has a certain amount of power from those huge solar batteries - and thus expecting at least some propulsion capability is due).
Unlike ww2 Japanese balloons, which could rely only on fragmented weather science and mechanical time fuzing - here it's stupidly simple to make the thing satellite-controlled (and also upload everything it collects in real time).
Moreover, those ballons are actually remarkably cheap to make and launch - those can be launched in thousands for no economic strain - and, thanks to lower altitudes, comparatively negligible drift(compared to satellites), and extended time over the target - even cheap recon payloads (optical, SIGINT) will be able to both get higher resolutions/amounts of data than ludicrously expensive satellites, and look underneath the anti-satellite roofs&covers. Exposition angles matter.
Potentially there is even the capability to release sneaky secondary gliders...or other gliding payloads, if one doesn't think that was serious enough. Bomber balloon may sound jokes and fun compared to a billion USD apiece stealth bomber ... until the world's most powerful military can't really do sh...t about it.
Overall, I am personally of opinion that (1)the thing is likely genuine, and (2)it appears to be the single loudest failure of US continental defenses since 2001. A thunderous slap in the face, all the more loud, because for 1960-80s NORAD it would've been a piece of cake mission.
p.s. at least watching self-calming by Twitter netizens was fun.
