Another reason to assume that Tau technological 'superiority' is nothing but propaganda is that we never actually see it in action anywhere.
Tau Barracuda fighters? About as good as Imperium Lightning fighters.
Tau Hammerhead tanks? About as good as Imperium Leman Russ tanks.
Tau spaceships? Usually inferior to Imperium spaceships.
Sure, the Tau Fire Warrior is probably a little better than an Imperial Guardsman on average.....but that's because the Tau bother to give their infantry the good stuff. If the Imperium bothered to give the Imperial Guard all Carapace Armor and the best rifles they can make they would absolutely be more than a match for Fire Warriors too.
Probably the only area where the Tau actually exceed the Imperium in anything is tactics. But that's about it.