I was told by two different tire places that my wife's car (2015 CX-5) uses the ABS sensors to detect if one wheel is spinning differently than the others. A flat tire rotates faster than one full of air because it's smaller.

I had the warning light come on twice recently: once, I pulled over to the shoulder when an ambulance was approaching. As I accelerated to get back on the highway, the warning light came on. I thought I'd run over a piece of glass or something. Using the gauge showed nothing wrong. Reset the TPMS.

Later, we were starting on a 30-mile drive down a gravel road in Eastern Oregon. The TPMS light came on. I took a deep breath, kept driving and after we reached our destination, I reset the TPMS and it hasn't come on since.

My conclusion is that I accelerated too quickly on the shoulder and some gravel or debris made a wheel spin faster. The second time, there was presumably more gravel on one side of the car and again one of the wheels spun faster.

My wife's car is 2WD. I wonder if either would have happened in an AWD.

Nah, it's probably not slight wheelspin because that would be a normal case that's handled by traction control. It wouldn't make sense for the system to both control wheelspin with traction control AND report it as a low tire pressure condition. Presumably the TPMS alert would be triggered by a longer term rotational difference. It happens for all kinds of reasons (eg, going into colder temps).

I have a CX-5 2016, and done things like overheated the transmission in loose sand. Never had the TPMS come on from wheel slippage.

My car uses ABS for it and the light goes out after something like 10 seconds of non detection above 30 mph.

You likely have an ABS sensor issue - the wheel-speed sensor systems don't get triggered that quickly.