He is saying that because WB was the strongest yonko, it only makes sense that he would have the strongest first mate. In OP, the first mate is usually the second strongest in the crew.
The Gorosei made a vague statement about the possibly of Marco being able to stop Blackbeard. The fact that they thought he stood a chance against a Yonko speaks to how powerful he is in theory. This was later proven to be false however, when BB destroyed Marco.