In the source page (http://taishoen.org/shopping-shoz.htm) there's a japanese heading that says "宮様楓" ... I did some digging in Chinese and they say that it's a term that is used for trident maples in Japan, though google translate gives me nothing for a proper translation, so I suppose it's not the official name. The Chinese written explanations say that they're tridents that had been exported to Japan from China... who knows...
I don't think it's as simple as just being a trident. Those leaves certainly aren't from a typical acer buergerianum. The lobes on this tree are super fat. After looking over some of the stuff in Vertrees' book, the closest match I can find is the acer buergerianum "Koshi miyasama."
But even still, it looks far different. The colors definitely don't match and these leaves look quite glossy.
I'm trying again - didn't realize you wanted the exact variety .. this page and this page would indicate that it's Acer buergerianum var. formosanum - that is, as long as the heading on the tree in the link you posted is accurate.