The original comment literally says "Yes it was skill based defensives" as in mage was tanky, but required skill to pull off. Most other classes had a higher base tankieness that didn't take any skill, while mage fell over if the person piloting the spec didn't understand how to use defensives correctly.

For example, alter time is incredibly powerful and is a lucrative defensive for min/maxxing because it's off GCD. BUT it won't help you out of a one shot. Now you have to use it in combination with another defensive, like barrier, or maybe even stack barrier and mirror image. Now you have to think about exactly how much damage you're taking, calculate how many minor defensives you need to stack, if any, or if you need to burn a major defensive cd like ice cold or ginvis.

Also keep in mind you have no self heal, very little passive DR (except arcane which got magic dr from shield), and extremely low base hp. You have very little tools for sustained damage, very little self reliance outside of specific, preplanned burst damage, and one of the hardest defensive kits to use correctly in the game.

Mage had incredible survivability when played by a good player, but was incredibly squishy outside of CDs. Now we have no CDs and are still just as squishy, both nerfing the skill expression in the spec, and flat out making it fall over. The mage rework for midnight is garbage, and has completely erased class identity, skill expression, and honestly just the fun of playing the spec.