Where does that idea come from? Models can generalise well - if you request photorealistic animals dressed in something specific, you'll get it even though there's likely no training image with that example. People wear enough close fitting clothes that the general form is easy to find.

I first heard the "we need naked images to generate good clothed images" when SD3 came out and suddenly it's everywhere. But it just doesn't make any sense to me and as far as I know it wasn't explicitly practiced in previous popular models.