I didn't really understand that, since there is a clear influence, but it is common to refer to third wave ska and ska punk as just ska, because shorthand. Hence my confusion I guess.
I guess it is confusing if someone isn't familiar with the bands that are being discussed though. Even within ska punk, there is a lot of variation. Band with horns, without horns, with clear ska rhythms on every song, with only some on some songs. Confusing all around I guess, but that goes for most genres/sub-genres. I find myself just saying punk and meaning post-punk, hardcore punk, pop punk, ska punk, garage punk, post-hardcore etc. I guess some find it tedious to try to be precise with the imprecise genre labels, which I find understandable.