He's stretching to find an explanation but doesn't quite get there. It's an interesting question, but this isn't a definitive answer.

Because if you have a 15% minority X, and you need to include it in an ad to appear "inclusive", then unless your ad has 9-10 people in it of which X is just one person in the ad, X would be over-represented.

So, if the average ad has 3 main actors/models max, and there is a 15% minority X, then even including just a single actor/model of X in your ad, X would be over-represented as 33% (a whooping over 200% over its actual share).

It's not a big conspiracy, just math. If the minority has a 40% share (compared to a single 60% majority) then it would be much easier to not over-represent it.