I mostly agree with you, in that we shouldn't conflate cross-browser support with cross-platform support.
That said (at least in the general web dev sense; can't speak for this specific project), it would be more accurate to say that it runs on three 'platforms': blink, gecko, and webkit (sometimes even trident!).
Again, that's a confusing way to put it, given the general understanding of 'platform' in most contexts as 'operating system', or 'environment'. But it describes the functionality more comprehensively than saying it only runs on one 'platform'.
That said (at least in the general web dev sense; can't speak for this specific project), it would be more accurate to say that it runs on three 'platforms': blink, gecko, and webkit (sometimes even trident!).
Again, that's a confusing way to put it, given the general understanding of 'platform' in most contexts as 'operating system', or 'environment'. But it describes the functionality more comprehensively than saying it only runs on one 'platform'.