I wasn't saying people have similar level of haki. The main thing i don't understand however is why do you need a stronger haki than someone to bypass his defense? I mean, if it's really the case, then does it mean that if you encounter someone possessing stronger haki than you, you can't beat him? Even if you have more fire power, physical strength, speed and the lot over him? I gave the Luffy-Gorgon sister example to emphasize that main pointI honestly said don't get what you're confused about nor where you see a loophole as if it wouldn't be like this, haki would make no sense and then you would have quite a few loopholes.
It doesn't really make much sense to say, which is pretty much what you just implied, that all haki users are equal. That just by knowing haki you now suddenly are equal to all the other people who know haki. Of course that's not the case nor did the series ever said it was like that. When two haki armament users fight, the one with the strongest armament wins. It's that simple and logical. It's no different than for instance two sprinters having a sprinting contest or two brawlers having a strength contest. The fastest will win the race and the strongest will win the brawl in a direct confrontation. So yeah for Zoro to be able to cut Pica's full body armament haki, he had to have stronger armament haki. The fact that Zoro was able to cut him simply means that his haki was indeed far more powerful than Pica's. There is really nothing odd about it nor is there a loophole. If Zoro's haki was weaker, he wouldn't have been able to cut him.
Armament haki functions like an armour and and every armour can be broken. Zoro's armament haki was stronger, so Zoro cut Pica's armour open. Every armour has its limit. It's not like that having armament haki you are suddenly immune to all damage. Do you think for example that even someone like Doflamingo would survive if a whole mountain falls on him? Having full body armament haki is not going to help you there despite the mountain having no haki at all. Haki has a limit. Same thing when Luffy fought the Gorgon sisters. Luffy was able to generate a large amount of strength which exceeded the limits of the Gorgon Sister's armament haki. The attack was stronger than the defense, thus the defense broke. In practice however it's just very unlikely that someone can generate enough strength to break the armament haki of a haki user while not being a haki user themselves. Most people who would be able to do such a thing, would have already learned how to use haki by that point.
Luffy displayed some lackluster performance in armament, mainly compared to Doffy and theoritically he should beat him by the end of the arc and it's what makes me confused about the whole "your haki must be stronger than mine" thing
Pica basically implied his level of mastery was high enough that attacks like 1080 Pound Cannon or Ichidai sanzen sekai couldn't hurt him unless you applied your superior haki in it? But if it is the case, then i can easily see him surviving falling from a mountain considering the level of attacks he was implied to completely block are mountain busters