Yes, of course, thermometers must be kept out of the direct sun. I have one outside my kitchen window facing southwest so I can see the air temp in the morning. On a summer afternoon that thermometer has been in direct sun for hours and I've seen it read close to 120F. The sun's radiation is heating the metal of the device which holds far more heat than air does.

So any talk about ambient temperatures is always assuming thermometers are in the shade. It is meaningless otherwise.

Correct.
I do not claim to be an expert , but I think In case of tomato plant, it is also median/average air temperature. Plant itself will not get that hot because it has a cooling system. And that depend on the Relative Humidity. But high temperature will stress out the plants that is why they will shut down production.
There are also genetic factors. Plants can adapt to therie environment in the LONG RUN, just like desert plants.

Thanks.

But if the sunlight is directly hitting the plant for 6-8 hours, isn't it true that the temperature of the plant will itself be higher than the air surrounding it? Is the plant's temperature relevant when determining optimum conditions? Or is it the soil (root temperatures) that we are talking about when discussing optimum conditions?

Both. Particularly when fruiting, the dark red skin of the tomato will absorb sunlight. This can cause sunscald.

Growing in such hot conditions can be challenging. It may be that you need to shade your plants.

The temperature of the plant in the sun can be higher or lower than air temperature. Being in a dry climate I'm sure you appreciate the cooling benefits of evaporative cooling. Water is evaporating from the leaves and cooling them. One way to measure the water status of plants is by measuring the leaf temperature with an infrared thermometer. A high leaf temperature relative to air temperature indicates a plant that is water stressed.

The 85-86F for tomatoes is air temperature not the temperature of something, like your thermometer, directly exposed to the sun.

Shukran Jazeelan (Thank you very much) for your responses.

This is why the majority of tomato cultivars will not make fruit in florida during summer, where the average daily high is a little above 90.