Literally just scroll up, from your post, for an example of their Mary Sue status. Melting a space marine, let alone a Chaos space marine, with a single burst of ion fire isn't Mary Sue status? Okay.
Space Marines are just bigger tougher faster humans in Power Armour. As tough as an Ork, as quick as an Eldar. They're not god-warriors. They die just fine. It's the combination of strong, tough, fast, skilled,
and armoured that make them the best, rather than having one or two like Orks/Eldar.
Also, if you take the current version of the tabletop rules:
A Tau Fire Warrior (BS 4+) is armed with a Pulse Rifle (36" S5 AP-1 RF1) and shoots at a Space Marine (T4 W2 Sv3+).
The Fire Warrior has a 50% chance to even hit the target to begin with. Then comparing S5 vs T4, they need a 3+ to wound them (66%). Assuming they wound, the Marine now gets a 3+ Armour Save, which would be modified to 4+ by the Pulse Rifle's AP, but Marine Armour ignores the first point of AP so they still get the 3+ (33% chance of the wound sticking). Now assuming that fails, you've dealt 1 Wound to the Space Marine. He has 2. Do it again.
0.5 * 0.66 * 0.33 = 0.1% chance for a given Fire Warrior to wound a Space Marine with their rifle
once.
So no, Tau are mechanically fine. On the other hand, GW writers have spent the last 10 years believing the shit they write about Marines and crawling up their asses. Let's not even start on Primaris Marines, the Suest of Sues.