This is the quote in question.
Quote
(Sandalath) ...Some Andii survived. And it seems there were more than just K'Chain Che'Malle dwelling in that region. There were humans.
(Withal) The Shake.
(Sandalath) People who would become the Shake, once they took in the surviving Andii.
...
[theory about all the Andii leaving the Shake to found Bluerose population]
...
(Sandalath)... The taint, Withal, of the KCCM. And so, before we Tiste even arrived, they lived in the shadow of the Che'Malle. And it was not in isolation. No, there was some form of contact, some kind of relationship. There must have been.
DoD, UK HC, p. 228/229
Admittedly, she is just guessing, but it never occurs to her that the Andii might have come FIRST on that shore. And it well should, if it was right out impossible that humans existed at that time (as we were supposed to think).
More:
Quote
(Bottle) .. [the Eres'al] conquered the entire world.
(Quick Ben) So what happened to them?
(Bottle) I don't know. I think, maybe, we happened to them.
DoD, UK HC, p. 225
Again, he is guessing, but again QB and Bottle have both shown an extensive knowledge about everything.
Quote
(Toc to Tool) After the Ritual, well, you chose the wrong enemy for your endless war of vengeance. It would have been more just, don't you think, to proclaim a war against us humans.
DoD, UK HC, p. 486
This is not even guessing. It clearly states that humans existed of the time of the Ritual and Tool doesnt deny it.
So my understanding is that SE drifted from the original idea of humans evolving from the last remnants of the Imass after the ritual (as it was implied in the earlier books) and all that "Founding races" thing (which was compromised as far back as putting TTT into the story and then even further with the Eres) and settled for something more closely resembling our own history. As has been discussed before, the Imass resemble Neandthertals, while humans resemble Cromagnons (Homo Sapiens) with the Eres the common link before them (Homo Erectus or whatever). On Earth Neanderthals had strong populations for a time and for a time they shared the world with Cromagnons but eventually died out and probably SE has used this very same analogy for the Imass and humans on Wu but indeed I am still a bit surprised he would go as long back with placing humans as the dying out of the Eres and the KCCM.