And it's a valid opinion, in my opinion. One could argue that the Unthanks are folk (or folk-rock). But there's nothing more or less rocky about Lake's original vocal--clearly King Crimson in general is more rocky, but not the vocals in particular, imo. Does "Islands" by KC contain a rock vocal?

Wyatt, though he is not clearly "rock and roll" or "hard rock" et al., is more rock than any other genre (and his second solo album, is titled "Rock Bottom"), though he might be considered "art rock" or jazz rock" or some other sub-genre. If Pink Floyd (for example) is rock (sub-genre psychedelic), then Wyatt should be as well.

If what designates a vocalist as a "rock singer" is that the songs of that performer are rock, then we have a number of difficulties, for example: "Lucy in the Sky with Diamonds" is a "rock song" but is William Shatner's performance of it a rock vocal? We also get into that very squelchy task of defining rock itself.