I bet you'd find a lot more variety in a random sample of 100 people today than you would in, say, the 17th century. I think the way he's framing this is weird. He's comparing drawings of aristocratic costumes that were notable enough to archive for posterity, as well as pictures of people's ceremonial garb (and best going-to-meeting clothes) with an unevidenced claim that people today all dress alike (do they?). To make an apples-to-apples comparison, compare those historical clothes with NY Fashion Week, or Cosplayers. Or, compare the surviving wool trousers and tunics of a medieval peasant with a picture of what people today are wearing in the average subway car or grocery store.
I think that the difference might be that today's aristocratic costumes and ceremonial garb is largely boring. In both cases they are largely just suits or cocktail dresses. Even the more unique ceremonial garb that does exist (e.g. robes in high church congregations and academic robes) are worn very infrequently today.
