Player inputs are not the same thing as champion outputs. Maybe Riot's done a historically bad job of making fun player inputs for tanks.
no (edit: one) interesting buttons to press. His output is clearly tank since 3/4 buttons apply crowd control and his passive and W give him durability. But he's not fun to play (for most people). I'd argue Rammus fills a similar niche in a much more interesting way.
K'Sante, however, has really interesting player inputs. He deals some damage, sure, but every button is CC or shielding. He probably "feels" like a fighter because he has fighter inputs: Tight timing windows (W), auto-attack resets, low-ish cooldowns, and huge positional play. So he PLAYS like a fighter or skirmisher but he still fits team comps like a tank.
Honestly, taking a slightly optimistic view of it, that just seems kinda genius IMO.