sm8ps is right. It is commented in sceconfig that the UPDATE variable overrides the PRESERVEDEBINXMODE variable.
But on looking at it, those really don't need to be in there as there are only a few times one would want to not fetch a new DEBINX and that would be easily done with the -p option. Such as back to back import sessions in a short time, rather than having not fetching new DEBINX as a constant.