Wow. I've heard the human brain struggles with 'of', didn't know 'if' caused problems too...
In terms of exposure, rap as a genre of music has a massively high percentage of talentless 'artists'. I doubt many would disagree with that. Hence the parenthesis. Simple.
Lyrics, definitionally, are to be sung. Categorising rap vocals as singing is at least as problematic as categorising a lot of rap tracks as music. Very much a borderline case. Hence 'proper'.
For the record, I'm not big on rap generally, but I do quite like a number of things which I don't refer to as "music" or "lyrics". Ice cream, for example, is quite plainly not music, but I still like it. I don't consider "it's not music" a derogatory expression.
Also, what Philly Q said.