The requirement hasn't been enacted into law yet (and I'm not sure it will be). However, requiring a statement about mentorship in the proposal and in the annual reports does a couple of things: (1) it forces the PI to at least pay lip service to the idea of mentorship, and (2) it sets up at least the distant prospect of enforcement. I can't imagine that anything ever would be enforced, but still. The idea of defining what an undergraduate education should involve has universities all nervous about academic malpractice - I could see this kind of requirement creating the same kinds of concerns.

If this were to go through, I would guess that it would create a demand that the university level for postdoc offices to provide basic professional development services to cover institutions' backsides, and that would be a good thing.