It clashes with established canon in more than a few places, so I'll try to sort out a few of the wrinkles. If I messed it up elsewhere, let me know and I'll try to bullshit my way through it address your concerns.
There are books in the Library that haven't been written yet. There are books in the Library that were never, will never, be written.
They were destroyed in other universes/realities, says I.
Q: What do Ways look like? You mentioned that every shadows or mirror could be a Way, if not a useful one. What else can be a Way?
A: Mann: An arch. A hole in a wall. A cave entrance. A picture of a door. Pretty much anything else you can picture as a door.
Mr. Jadun is probably incorrect in his theory. He is writing from his own perspective, as someone whose "door" to the Library was his own death (it's a metaphorical door, shut up!) and the destruction of his personal library.
Also, if this is true, there is an entirely reasonable explanation of how the Journey of the Walk is updated.