1) Luffy wanted to learn barrier to get through the scales, he went beyond by acccident, meaning it has always been barrier haki that is required, Luffy learned penetration because it is stronger
2) This also only applies to physical damage, as Kid was literally pointing out this chapter. Every other damage was never part of Kaidos durability, his scales are so hard physical blows, even with Haki, do not get through them. Any other damage is anybody's guess.
3) The pic does not show the only people who can fight him, it shows possible levels and their representatives Luffy could develop to who Kaidou has encountered before, basically, Luffy belongs to the people who can fight him, but who can he match of those who could do it, thats the context.
Considering Kaidos statement is in regards to Haki as requirement to fight him, we already know those 5 are not the only ones because we already saw 9 people in this arc who could, they were just too weak besides that.
4) The motif behind the thread is obvious.