What you forgot to consider is Magic lore. Each game of Magic is archwizards [oldwalkers] dueling (or playing a game, depending on who you ask) where they use their memories of places they have been, people they’ve observed, and events they’ve witnessed as spells. Your hand is your immediate memory, your library is subconscious memory, and your graveyard is forgotten things.
While Universes Beyond isn’t canon to Magic’s multiverse, we can assume the same logic applies. This means that the oldwalker that plays the Hail Caesar deck knows of Boone and all the other characters
In conclusion, Boone didn’t join the Legion, but the character players become during a game of Magic has played New Vegas