Yeah, so with you using LED it doesn't make much difference. But with the standard grower its a rule that seems to have been forgotten.
The temps that are most often talked about and most vital are the air temps, so you would measure it in shade and with well mixed air, its often stated above the light line, but just as well in a shaded area where air is not stale, you could get a difference in air temp from lower to higher in a situation where air is not been moved.
Anyhow the idea is to have something like a Stephenson screen, which reads air temp without interference from light radiation.
The reason we measure air temp that way is too be reasonably accurate, we can use our hand to measure radiant heat (again this is more specific to HID lighting). Plants have a low tolerance to high air temps, it greatly impedes performance, but radiant heat is different, plants can take almost double in radiant heat, some over 50degrees C depending on cultivar. So we use our hand at canopy to measure radiant heat and if you can hold your hand there its a pass. Obviously its a crude way of judging but it works as a rough guide.
I've seen plenty of folks panic and prepare to give up over the years because they had a 600w HPS in a 4x4 and a black hygrometer at canopy level and cant figure out why they are running at 95f no matter how much extraction they are using. Luckily most hygrometers are white so the effect is lessened, but it still throws off the readings and the dialing in of the environment.
Someone may think they want to run at 74f yet could actually be running at 69f and wondering why plants are not transpiring properly or why they have slow growth and a calcium deficiency and root problems.
That's my 2cents anyhow, excuse my rambling.