Keep in mind that this is a beginner series.
It's aimed at preventing beginners from ruining trees by watering the foliage too much.
Photosynthetic ability is lost when a lot of dry dust accumulates on the foliage. So hosing it down once in a while isn't bad. Doing it every day, or more than twice a day can leave the bark too wet, possibly introducing fungal issues, algal growth and washing out antibiotics or pesticides.
Apart from knocking off branches, causing salt buildup from evaporated water, and all those other things that could go wrong.
If you have to lay down absolute rules for beginners, I think it's better to play it too safe as opposed to leaving the beginner to fill in the gaps with their own sometimes twisted logic.
It's true that wet foliage evaporates less and (cold) water can close the stomata, causing reduced respiration. If there is a lot of wind and dry heat, the plant can cook itself when that happens. But it's a rare occurrence, usually the water on the foliage acts as a cooling agent itself. But if it does happen, and Ryan advised it, then he'll lose credibility as a teacher.
If his teaching is his main source of income, then I can imagine he's advising to not take any risks and go with the safest route.
