So poppy isn't a 'hyper scaling' tank as you say, but she's a 'hyper tank'?
She's a hyper tank in that what defines a hyper tank is the same as what defines a hyper carry. A hyper tank has the maximum bulk possible, that doesn't say anything about actual scaling. It's the same for tanks as carries, even though Kog'maw's a hyper carry in practice he's outscaled by many ADCs who you wouldn't consider hypercarries due to the superior value of their utility or mobility. It's the same for Poppy, she's a hyper tank but in practice she's outscaled by some tanks who aren't like Sejuani and Amumu due to their superior damage.
If you're arguing for her as a hyper tank I think that's fine, the word which I disagree with in your statement is scaling. She's not a tank who scales up well at all, her spike is in the mid game and she's quite possibly the weakest of any tank in the late game. The only tank I think I wouldn't take over a Poppy at 6 items is Galio and that's only because his kit is an absolute clunkfest right now. I'd take any other tank over a Poppy at 6 items, even someone like Leona or Alistar is far more reliable.
I get that her damage falls off as the game progresses but she is a complete tank in the sense that she can self-peel, peel for teammates, engage, disengage, isolate, 1v1 squishies
All of her abilities have heavy reliability issues. She can only really play out a fight if she can either flank or kite it out. When she's forced to dive she often has to E into no stun and that normally isn't good enough. She also lacks true AoE CC, her R almost is but the fact that it makes enemies untargetable makes it far worse than abilities like Depth Charge and Glacial Prison.
She's far from a complete tank, she's a tank who mostly only functions when kiting back. If you try to chase into a Poppy she tends to be super strong but she's probably the weakest of all tanks at engaging on the frontfoot.
and her resistances/defensive abilities are very powerful which is akin to Maokai and Nautilus who you agree to be 'top scaling tanks'.
I actually didn't say Mao and Naut are the top scaling tanks, they're the top scaling tanks who are like Poppy. The absolute top scaling tanks aren't like Poppy. Sejuani, Amumu, Zac Sion and Shen all scale harder than all 3 of these champs, in the first 4's case they do so by being massive AoE damage and CC threats and Shen just becomes one of the most reliable splitpushers and counter engagers in the game, with the right comp he's also an incredibly strong engage champ.
The thing is because damage is so much better than durability simply having a ton of durability isn't the optimal way to scale as a tank. It's great in the mid game when Mao, Naut and Poppy spike but into the late game tanks who can deal damage quicker and better than Poppy thrive. The thing about hyper tanks is that they fall off. The closest thing in the game to a "hyper scaling tank" is Sion, but he's not really a hyper tank in the same sense as Poppy, you can pretty much focus down a Sion in a reasonable length of time at any point in the game, he doesn't have the mid game unkillability spike the other 3 have.