If BB gave Shanks scar during that time. honestly, Oda would be a weak writer, or to be more precise, he would be someone who wants to please the Shanks fandom and doesn't have the balls to make them mad/upset.
Because why would Shanks who was still a newbie, still an apprentice, to actually say that Blackbeard is hiding his true power and living in the giant shadow of the so called WB. And saying he is more dangerous than what he tells you, if Shanks scar was when they were still weak and kids.
MAKES NO SENSE
Shanks is like: I was not letting my guard down, Whitebeard. Do you get me?
I mean, why would he warn WB about him and his power, and that Ace is in danger, IF Shanks was talking about Teach's level when they both were an apprentice. Makes no sense, and Oda would just be pleasing Shanks fans by not making the scar later when Shanks became more infamous.