You missed that entire discussion about the changing nature of the wall of the faithless didn't you?
The actual existence of the wall is entirely unknown at this point. It was turned into a mirror during novel events, only to get revoked by SCAG, which then had it's entry for the wall of the faithless get removed in an errata.
As far as we know, it's still a mirror, until another more recent sourcebook comes along saying otherwise.
