Instead of making it a world to travel to, they want to make everything complicated to make it hard to travel to even though Jefferson went there in his hat, so it must simply be another world that runs parallel to everything.
So, to put it in show terms and simple terms....
If you travel to Camelot in the past, present or future, you could all arrive there at the same time. Camelot is "outside of time". That means that while their story happens, you can't place where it happens because the story is still going on.
To the person of the Past, when they return to the past, Camelot's story happened in the past.
To the person in the future, when they return to their future, Camelot's story happened in the future.
To the person in the Present, when they return, Camelot's story just happened.
So, it happened in the past, present, and future, because it lies outside of time and runs parallel to all time periods.
Or maybe they made something up and they probably are right with you and the rest of us as confused.