It's a display box! It's for showing off the watch. I've seen similar where the box is basically a posh block of wood with a hole in it for a perfume bottle.
I am well aware of what it is and it’s purpose. As I said if the maker and intended user like it that is all that matters, it is beautifully made and probably beyond my skill level.
None of that changes my opinion about the wall thickness for a box of those dimensions.
in the past I have made boxes using similar thickness material that are probably 2 to 3 times larger and the sides still look too thick.
Please remember that the thread is about stock dimensions for making boxes. Structurally, depending on the material, for a box of that size you can almost certainly go down to 8mm or less but at under that thinness it may look too thin.
You can use almost any thickness you like but since today many people can only get S4S in certain thicknesses and they don’t have the ability or desire to change that you will often see boxes that are more substantial than looks as good as they could.