Short answer: we don't know much at all about the Concordance of Rivals -- all that's confirmed is that it's a powerful artifact on par with the Book of the Damned or Chronicle of the Righteous, written by the same author.
(Moreland's inclusion of the Concordance alongside other artifacts in that tumblr post -- which predates the release of Hell Unleashed -- may actually have been a minor slip of the tongue, as the book had never even been mentioned by name in official material prior to that Campaign Setting book.)
It's within the bounds of possibility that the Concordance and the Lost Gospels are one and the same, but unlikely given the nature of the Lost Gospels -- they weren't actually delivered by Tabris at all, but rather by Aldus Canter, founder of the Esoteric Order of the Palatine Eye. (The Lost Gospels are more or less analogous to the irl Book of Mormon; Aldus claims to have received knowledge of its rites and mysticisms through direct communion with Tabris, but these gospels were only ever relayed through Aldus himself, and the true author is unconfirmed.)
In my opinion, it's more likely that the Concordance of Rivals is Tabris' account of the neutral outsiders, and of the tenuous multiversal balance between law and chaos. If the Book of the Damned is
One thing is for sure -- we could really use more information about this tome! Anyone know a good way to bring the developers' attention to this issue?