You properly could identify cars uniquely by the sound they make. If not now then soon.

Having worked with the result of prior generations (circa 2010) of algorithms for that sort of thing in the radar spectrum (I was not privy to the actual algorithms that underpinned it all) I suspect accuracy drops off exponentially once you get away from text based stuff and flagrant body differences (missing mirror, aftermarket spoiler, etc).

Note that Flock has incorrectly identified several vehicles as "suspects" to police during investigations.

Flock has two obligations. Sell equipment to police. Avoid freaking the public out.

Their statements are almost certainly not reliable.

Absolutely they can. Vehicle panel colors, wheel rims, roofrack, tow hitch, bumper stickers, damage all factor into their vehicle fingerprinting.

And once they've got a real license plate for the vehicle, all the historical information for that vehicle fingerprint's activities are now linked.