No, they didn't overestimate themselves, they just that powerful, simple as that. Pairets in general are more powerful than Marians, for example; WSM, WSC, WSS are all pirates then you have Roger, Rocks, Big Mom, Shiki who never feared an Admiral, were never defeated by an Admiral, Shiki even couldn't believe that Roger was captured by Marians since he new that Roger was too strong for them.
Blackbeard said to Whitbeard that he was respecting him but he's too old and can't protect even his crew members "meaning that healthy Whitbeard could defeat the Marians"
Orochi who is insanely caword dude but at the same time
smart enough to know how strong Marians are says "I have Kaido backing me up" while threatening WG directly.
Big Mom and Kaido are speaking through open line whiteout giving a sh#t about Marians
Big Mom doesn't even Consider Marians as threat to her
Yonko are officially the most powerful pairets and I know that you going to say "that it isn't about strength it's about military power" but that doesn't matter since personal strength is also part of military strength.