I'm not onboard with that theory.
This seem very clearly telegraphed to be about someone we - the audience - already knew, i.e. Roger.
Roger and Shanks' past with him is of course is discussed almost immediately after.
Whatever level you think Shanks is, I don't think it can be taken for granted Garling is even all that close. He may very well be, but I think we can pump the brakes on who can scar Whitebeard and who Oda would go out of his way to foreshadow
652 chapters prior to their debut.