The pair of ghosts say they met Mew, Gastly, presumably Ash 2,000 years ago, isn't that hard to believe, Since Dialga and Celebi can time travel, So it's not out of the question to assume he time travels to the past in the future. Time travel is always confusing when it comes to Butterfly effects, Paradoxes and Origin points etc.
Part 5
The pair of ghosts say they met Mew, Gastly, presumably Ash 2,000 years ago, isn't that hard to believe, Since Dialga and Celebi can time travel, So it's not out of the question to assume he time travels to the past in the future. Time travel is always confusing when it comes to Butterfly effects, Paradoxes and Origin points etc.