Skirts have a fair amount of airflow, which would be cool ambient air. An occupied nylon tent would have people exhaling ~80-90 degree air into it all night, even if the rest of their bodies were perfectly insulated.
Except the tent wall is:
1. Opaque to LWIR
2. Very low thermal mass
3. A near perfect convective surface for the outside air
And tents are designed to be ventilated
While I think it is likely a tent wall would be slightly warmer than ambient if occupied, I would also venture that the temperature difference is much less than expected.
Since most of those imagers are auto-gain controlled, and 8-10bit resolution, if there were any warm objects (hot pot, fire, possibly even bare skin) in the image, the normal operating mode would set that as the full scale readout (reduce gain) so the minimum detectable temperature difference (least significant bit) could be as low as 0.5 degC (120degC full scale, 8 bit). Normal convection coefficient for air is 10-30W/m2-degC and the resting metabolic rate of a person is ~75W, so with the surface area of even a sealed small tent (say pup tent triangle style, 2m long, 2m at the base and 1.5m tall, or a surface area of 8m2, the temperature difference would be 0.3-0.9degC relative to ambient,or barely detectable.
Thermal doesn't see through walls, and low mass, opaque sheets are simple LWIR blocking surfaces when exposed to ambient air.
However, besides the technical point, I would bet the conclusion is accurate and more tents are used than people to inflate the protest attendance