Sorry but that's complete rubbish. Particularly in 4th edition, Tau was usually Mech, because of skimmer rules. Tau gunlines are products of late 5th edition mindset when players had to squeeze out maximum firepower to stay competive against newer more powerful codices.
Then you havn't played third edition tau, which was a gunline army at the time, with the skimmer tau being because Skimmer Rules were so blatantly horrible, that Eldar was pretty much the [Insert Broken Codex] of its day due to it, along with Tau and it's "Fish of Fury".
It was quite simply several broken Skimmer Mechanics that made it mech, if they had at the time had normal mech rules, they certainly wouldn't have been a mech army at all.