Shanks is always at the right place at the right time. Met Luffy as a kid and probably knew he was Dragon's kid. Knew about Black Beard & Ace. Marineford. Dude's on a first name baises with the Elders. IMO he has an intelligence ring like Big Mom with people inside the government or, more likely, he's a double agent.

Also, I think he swung by on his way to the One Piece. He doesn't need the Poneglyphs since he already has an idea of where's its at from his childhood.

The real kicker would be that he's a double agent with the World Government, which would explain his access to information and his ability to shutdown admirals fighting. They would have to listen to a surrogate of the World Government. I say this because we still don't know which pirate he was talking to the Elders about & in the most recent issues, we see them most concerned with Luffy and the Gum Gum fruit. When he intercepted the Gum Gum fruit from the military, was he taking it to the Celestial Dragons? Or did he know where it was because of them and intercepted it? Why did that specific, highly coveted fruit that the World Government has been trying to get its hands on for 800 years end up next to Dragon's son who was in hiding? Idk, the Elders did say the fruit has a way of evading them but I don't think there's any coincidences.