They never call it handled. It's a portable console.
I guess everyone has its own definition. Mine is it is a handled console if you can comfortably play with it in your hands during 1h without arm/hand/joints fatigue.
It would be called "pocketable" or something like that otherwise. I use my Steam Deck and Switch exclusively by holding them in my hands and never dock.
So can a 15" laptop. That doesn't make 15" laptops handhelds.
It seems strange to say something needs to be pocketable in order to be considered "handheld".