It's not just a Van thing that death mages can't use other magic. When people try using the demon king fragments they lose their ability to use other attributed magics. Like after Gubamon is defeated it's pointed out he would have been much better off using his space attribute magic and martial skills rather than relying on the demon king fragments, which carry a lot of downsides to use. I don't remember it being said that he has no elemental aptitudes. He has no cheat ability and no outrageous talents for magic the way the bravers do, but that doesn't mean he can't use normal magic at all.