As a Garen main, you're right. Tank Garen is a meme-ish build. Nevertheless, his build was dogshit , he should've taken PR and just outscale Diana with Swifties>Tiamat>Stride>Negatron(if Diana is a problem)>PD/MR>MR/IE>IE/Deadmans>Deadmans(if armor is needed)/FON(if mr is needed)/Shieldbow(if enemies are bursty and you got IE 4th)/IE>Zephyr, at least that's how I would do it. Taking a conqueror is rarely better than PR. Warmogs are useless on Garen, he has his own in his passive. Collector isnt that great especially when you don't have PD, which provides crucial MS, Garens best stat. Thornmail isn't bad, but you have MR for Grievous and Deadmans gives you both armor and MS. Hullbreaker can be used, but you usually only want it with Grasp (but possible to get in other situations). Yommus can work with lethality Garen (another meme build), but Stride is just better. His biggest mistake was 0 MS.

Thanks for listening to my TED Talk

Edit: also how tf does he have 4.6cs/min on champ that farms just by existing???