>Clean doesn't really mean anything in design

It actually conveys pretty well that the interface is uncluttered and with a minimal aesthetic, with lots of space, etc.

>Are all other interfaces that are not clean, dirty? Or unsanitary? Or cluttered?

Obviously cluttered.

Don't really understand the objection. "Clean" has been part of UI vocabulary, and before that, of aesthetic criticism (in fine arts, architecture, industrial design, etc) for ages. ("Clean lines", etc).