Maybe I just misread it, but when the Lizardmen pack up and leave it wasn't all of them, was it? If I remember correctly, the Slann took off with a number of Skinks and some of the Lizardmen, but it sounded like alot of the Red Shirts were left behind. So technically in 9th Edition, you could still run an army of those left behind after the great Slann Rapture, you could even argue that a Slann or two was left behind either as 'rear guard' or just not getting to an Ark in time.