When did I say they can do what Shanks' just did? Nobody except Roger, the Conqueror of Conquerors, was my argument.
Nope, that's just copium. They have been portrayed to be similar powerlevel wise, which is reflected when they split the sky. Katakuri and Luffy clashing with CoC showed that they were similar in PL, not in Haki mastery (Kata was still superior). You cannot possibly convince me that Big Mom with her completely offpaneled Haki showings has the same amount of Haki levels and can do the same thing as Kaidou (All 3 Advanced Hakis) & Shanks (Best CoC and likely all 3 Advanced Hakis, at worst 2)