There's no forwards or backwards to what I posted.I think you have this backwards. Good players can win with most armies. But a more powerful codex would allow bad players to win more often.
The question should be,"will a bad to mediocre player win more with Orks or tau?"
You judge an army based on how one type of player can play it.
If an average gamer played all of the armies, against other average gamers, which one would he win the most games with? That's how you create a tiered list, because it eliminates the variables.
Will my Corvette beat nearly every Mustang around Nürburgring in Germany? Sure. Will I beat Michael Schumacher if he has the Mustang and I have my Corvette? Probably not. But the Corvette is still the superior car in every way.
However, if you give Schumacher both cars, what does he win more races with? Look at the tournament winners. It gives a pretty clear picture what the best armies are at any given time. It isn't like suddenly all the Grey Knights players forgot how to win games, and there was a Tau and Eldar Renaissance. Those armies are just better in their current forms.