I know is still in the air, but before you said is headcanon, three things:
1. Back in the days people though it was Roger, in the same conversation, WB mentioned Roger by his actual name, why say "HIM" so dramatic and mysterious.
2. After Garling revelation, people asumed it was him that WB refered. But others still claimed that Garling doesn't looks like Shanks at all. Then Oda drew young Garling and it was just like Shanks.
3. Too much coincidence for both young Garling and young Whitebeard being in God Valley. "But that doesn't mean they fought" chapters later Oda show how Garling was an active combatant too.