I don't believe the amount of damage you specified for one reason: Garen's Tier 2 ult should do 350 plus 33% of missing HP. And everyone has Magic resist, so the damage should have been reduced by about 30%.
One thing to point out: Garen is a Fighter. A Fighter is a Tank with assassination potential. Not as durable as a tank and not as destructive as an assassin, but could potentially become either if they build and play correctly.
The reason Garen is so strong right now is because Riot overloaded him with a third passive: the Villain Mechanic. And to top it off, they buffed his most powerful ability by making it true damage... Absurdity. I am cool with Garen having true damage on his ultimate, but I never expected Riot to give his ENTIRE ult true damage. I have always been posting to give him a partial amount.
What's dawning on me is how people are struggling to counter Garen. His early game is weaker now. Pantheon, Teemo, and many ranged champions can keep Garen off by poking him for days, forcing him to not farm and stay near turret. I had to fight Kennen and I could not farm effectively because she kept poking and stunning me. Kiting champions and ranged champions are the ultimate counter against Garen. How are people not doing this?