Help With Worldbuilding

Its my first time being dm for a campaign that isn't a pre-made campaign. I had it in the world's lore that dragons hadn't been seen in hundred of years and one of the final battles includes a horde of dragons coming over the mountains with a bunch of foretelling and so on.

One of the players is a ranger who wants to be a drake warden and he wants this for his backstory: "I was once a strong soldier I had the dragon back in the glory days and it'd be like reuniting with an old friend"

This works well for the previous backstory he has given me but it kind of breaks some of the world lore. How do I work this into the campaign nicely? Any tips are appreciated!