I could understand why he did that. Shanks isn't a fruit user, so yami yami isn't as useful and he barely had any control over the gura gura as well, so base stats would have mattered most in the battle. Not to mention, he said it was too early to fight you guys.
He didn't have just Shanks to deal with, especially when he was already having his hands full with Sengoku and that the rest of his crew was depicted to be inferior to Ace(so couldn't help much against Shanks' armada).
Not saying he could have beaten Shanks though, but Shanks decided to use that argument in order to convince even Whitebeard concerning BB's dangerosity, so while he may not have been as powerful as he is now, he was probably still significantly strong back then for his argument to be convincing. Don't forget he had the potential to be the second division commander without any devil fruit
