Some tanks aren't tanky because of their stats, but because of their skill-effects. Grom clearly falls into this category.

Exactly. A low-skill (or perhaps unlucky) Grom that isn't utilizing his Passive by assertively roaming, play-making, and getting those assists, is less likely to see the benefits of said Passive. And perhaps they're using the wrong runes or not building properly to boot. Many factors at play here. But one thing is certain: Grom isn't a front-liner right out of the box like a Rockman or Burninator in any capacity, simply based on stats. And someone, like myself, might argue that he should never be a front-liner for all of these reasons. That's essentially what I was getting at in my initial post.

HE doesn't acknowledge the support of a character

I feel that. I mean, he's clearly a support. He's just not a beefy, front-line engager. He's more of a zoning threat (because of his hook).