Ngl, I tend to not see that type of stage wear as fashion in the usual sense of it.

Stage costumes tend to be a thing of their own, outside of the kind of clothes you’d wear for real, and often outside of the arty stuff in couture.

At best, it borrows from fashion as an art, or street level fashion.

In this specific case, it’s more of the same. It’s costumery meant to refer to lingerie, not actual lingerie. In real terms, the distinction is meaningless. What clothes look like is what they are. But I ran across this post in a fashion community, so I figure that opinion (even if disagreed with) is relevant despite that