Where is written that Aurax gave the tusk to Angeshraël? I seem to recall that the Inchoroi gave the tusk to "the fathers of Men". This, if related in a mythical fashion, might refer to some forefather of Angeshraël. The concept of father of Men might have conflated different historical figures together. So, the continuity error (even if it exists) can be retconned simply interpreting it as an obfuscation due to oral transmission and mythical interpretation.
Obviously if Bakker stated this in some interview/FAQ or so, then such a retcooning would be much more difficult.
I remember that is is stated somewhere that the Inchoroi were pissed off by the slowness of Men to react to the message of the Tusk...so it might be that it took various generations before it was really absorbed and metabolized as scripture, way too late for the immediate need of the Inchoroi.
Obviously all my memories on the subject might be utterly wrong.
P.S. After writing all this, I have actually found the relevant quote here, with no mention of either Aurax or Angeshraël, (
https://fantasyhotlist.blogspot.co.uk/2011/07/r-scott-bakker-interview-part-2.html):
So the Inchoroi began giving them to the Men of Eärwa, hoping to incite them to rebellion. But the Halaroi had no stomach for rousing a feared, and most importantly, absent master, and so rendered the deadly gifts to their Nonmen overlords. The Inchoroi then looked to Eänna, where the Men were both more fierce and more naive. They gave the Chorae to the Five Tribes as gifts, and to one tribe, the black-haired Ketyai, they gave a great tusk inscribed with their hallowed laws and most revered stories–as well as one devious addition: the divine imperative to invade the ‘Land of the Felled Sun’ and hunt down and exterminate the ‘False Men.’
The Nonmen only rebuilt and reinforced the Gates after the first great migratory invasions generations later.