I dont understand the arguement over a tank vs a champion with tank items

So people yell at me (type in caps at me) when I call a champion like fizz or ekko that has built full tank items a tank. In my opinion if they are used as a frontline disruptive champion that is able to take some damage they are a tank. A bruiser falls under this catagory to me a trundle or garen will suck an entire teams focus onto them if they get even remotely close to an enemy carry so if the only damaging item they have is a black cleaver or titanic hydra what does it matter that Riot calls them a bruiser or diver. They are fullfilling the role of disrupting the enemy damage onto your tanky self instead of onto that precious kogmaw or vayne melting their entire team.