You're right of course in that 'Is a gelada a baboon?' is at heart only a linguistic question - 'can we call this a baboon?' - but it still remains that all we are trying to do is to get our terms and view of the world to fit with the Universe as we observe it. Which usually means taking more and more observations and trying to get a consensus view. With any data set, there are outliers - odd results - and if you happen to get a lot of these misreadings or extreme cases early on it can skew results. Which is why, as time goes on and more studies are done, our best guesses get better.
'Truth' is probably too much of a loaded word; but there is 'what happened' and 'our understanding of what happened', and all sciences (including taxonomy and systematics) are trying to improve the latter so that it better matches the former.
Blimey, this is deep for a Friday lunchtime!
Strictly, how things are related is systematics (which is more in question here) and taxonomy is 'the naming of taxa' - scientifically speaking, and as noted above, the word 'baboon' has no scientific significance at all. So the debate really is 'Do Theropithecus and Papio share a sufficiently recent common ancestor that we can reasonably apply the same English common name to them, without including taxa to which we certainly would not apply that name within the same group?'. And when the question's that awkward there's no wonder the debate goes round in circles!
