Where do these assumptions come from? lol. I think the only person who we've already seen that can even be considered near Shanks level who isn't actually a Warlord is Mihawk. Simply for the fact that Zoro is looking pretty beastly right now, he can probably take down some of the Shicibukai's, maybe I'm being too generous but he should at least be as strong as Luffy pre-timeskip, but he also has Haki so yeah he should be even stronger than that. Yet I'm sure Mihawk wouldn't have taught Zoro everything, then there comes the question of whether or not Zoro was capable of learning everything that Mihawk did infact want to teach him in such a short time. Then there's the fact that Shanks used a Sword, yet Mihawk is the strongest Swordsman in the world, maybe Shanks just doesn't classify himself as a Swordsman, or maybe when it comes to Swordsmanship Mihawk outclasses him etc.
Anyway it seems like a lot of people are making some pretty weird assumptions about Dolf. Just because Dolf has some really nice relations within the NW underground scene to the extent that he's often the center of attention, doesn't necessarily mean he's exceptionally strong. It just means that he's a good underworld figurehead who has several benefactors who may be monstrous. However that doesn't mean to say that he isn't abnormally strong, but it's too soon to say that he is.
