This is not a theory I came up with myself but one I heard.
The game takes place post WW in a timeline split where Ganondorf returns Hyrule to it's former state and removes the ocean with his wish.
Evidence includes rock salt in the over world with a description 'from an ancient sea' and the Koroks from WW which were said to evolve after the sea came in the first place.
There is also the Temple of Time from OOT which would have been washed away had this been in the regular adult timeline without this theorized split taking place.
They also theorise that Daphnes Nohansen Hyrule is in fact the old man from BoTW. There is certainly a similarity, the dark skin, white hair plus the old dude is very tall. In Ww Daphnes is almost as tall as Ganondorf himself.
I don't know if I think they will do further splits in the timeline but I wouldn't put it past them.