Garen is a tank simply because all of his abilities are centered around being a tank but unfortunately Juggernauts are broken except Skarner and they made Garen a juggernaut simply so they could change his kit and apparently this is a thing, see below:
Illaoi - High Damage Output, self heal, movement speed slow, average to high HP, can 1v3-5 if they know how to play and the enemy does not
Darius - High Damage Output, Self Heal, pull cc and a slow, Average to High HP with a true damage ult and an annoyingly strong passive can 1v3-5 if they know how to play and the enemy does not
Garen - Easily Kited, cc duration reduction/damage reduction, passive regens HP over time while OUT of combat, not like the rest of the Juggernauts, E shreds resistances as well whether it's for team mates or not. Ult can only be used once, more effective on the marked target does substantially less on everyone else, movement speed buff with a silence other than E, completely single target builds better if full tank and black cleaver
Skarner - Same boat as Garen builds better if full tank items plus black cleaver
Poppy - Tank, easily kited, dash protection, barrier, builds better full tank, stun if she hits you in the wall and a huge knockback, deals % damage on Q hit.
If you can't see the fact that Garen should have been a tank from the beginning(WHICH HE WAS AND HE WAS GOOD AT IT), Then I don't know what to tell you, and I can already tell you how this tank update is gonna play out, all the ones effected are going to have some abilities that convert resistances into damage stats just like a built in thornmail/Rammus W. More damage on a tank? What?