SCOTUS didn't say that Congress couldn't impeach. As I understand it, SCOTUS said that POTUS couldn't be prosecuted as a civilian for things he did as POTUS. This puts an asterisk on the "no man above the law", as they are saying that if POTUS does something impeachable but Congress doesn't impeach/convict, then there's no other recourse for holding POTUS accountable. Trump is taking advantage of that for everything that it's worth.