I think the key is "on a well-made/fitted kilt." The belt should be optional if the kilt is made to hug your body at the natural waist. Even then, some GoS (gentlemen of substance) find that a belt or braces/suspenders are required to keep the kilt where it's meant to be.

With an off-the-rack kilt that "kind of" fits, all bets are off and many belts are on!