To be fair to Arnault, he isn't just CEO of Louis Vuitton but LVMH which owns something like ~100 brands, ranging from clocks, wines, fashion, cosmetics, boats, jewelry and other retailing. Although to be fair to you, most of what they sell could go into the "luxury" category.

It's likely he wouldn't be the third richest person with just Louis Vuitton and it's probably mostly because of LVMH.