Human beings radiate IR do they not? Silly question, of course they do. Perhaps not above an Arizona level of thermal background?
That said, Arizona in the summer is going to play havoc with lIR and thermography in erms of false positives and negatives. The sensor suite probably should be using lIR at night for this reason and the switching it off in the day. But given Uber's history, the lack of lIR reeks of cost-cutting.
Air has such a low thermal mass that it doesn't measurably affect most IR sensors. Hot pavement could be a potential issue, but that shouldn't have a major effect on forward-facing sensors.
Besides, it's only March. Even Arizona isn't that hot yet.