It is headcanon because not a single panel even remotely comes close to confirming that, it's just an excuse to not lumb Shanks together with everyone else that was hindered by Kaido from reaching the One piece.
I am arguing from the more logical POV. Kaido goes down and suddenly Shanks says it's time to go after OP even saying he has absolutely no interest in seeing Luffy, yet somehow he waited for him so that he can start advancing ?
If Elbaf has a RP, Shanks automatically has access to it. They didn't just randomly cross Elbaf for no reason. Even Blackbeard is stated to have a RP given the narrative of Law vs. BB was that the winner takes all.
It's evident Shanks has a RP too and now the remaining parties, namely Straw hats, BB pirates, Kidd Pirates and Red Hair Pirates are going at each other.
It's not. It's cope to think Shanks has one when nothing in the Manga hints he does. Elbaf stands alone. If they protect a RP it's without Shanks since they have the strongest army in the world. They can defend themselves. If they choose to give Shanks a copy that's one thing but it still in no way means that anyone has to combat Shanks to reach the One piece while they have to confront Kaido or BM and so does Shanks.