People need to remember how pre-timeskip worked.
Luffy beat Crocodile, "supposed" equal to Mihawk, yet we found out soon after that was not true. Then he fought Enel, who was probably one of the most broken DF users outside of Kizaru for non-Haki users. Lucci or Moria would get rocked him. Yet, as progression showed, Lucci was far more dangerous to Luffy specifically because of his natural defense against Electricity.
So take this with Kaido. The most tanky villain in the series by far. He has fought 16 people and only "just" started getting tired when he fought Luffy on round 2.
- Does Kizaru need to be "tankier" than Kaido? No, he can just be stupid broken fast and requires the opponent to have mastered FS to keep up with him (makes you wonder why Oda had luffy start learning FS in the first place).
- Does Blackbeard need to be tankier than Kaido? No, he potentially has 3 of the most broken DFs in the series. He quite literally can make you lose your powers like Kairoseki, and he can destroy the world with his other fruit. Who knows what his last one is if he has it.
- Does Akainu need to be tankier? No, he quite literally may have the highest AP in the series. One wrong punch from him can kill you are permanently scar/maim you.
- Does Shanks? No, maybe he is the strongest "haki" user in the series (im just making shit up so you see what I mean, maybe he is, maybe he isnt).
- Im? Could have some other broken power
- Rocks? If alive, same as Im
You get the point
Tldr: It doesn't matter if Luffy is "deemed" the strongest to future villains