It should be left as subhuman or translate it as different human species or create something new like "other human" or "different human" or just use "ancient human". Demi-humans is something used in fantasy themed setting, and this story is based on our world, also Jevlenese are fully human just like Homo sapiens on Earth.
What I mean is simple, books are over 40 years old so the story is based on the knowledge from 70s and I think it is like that:
Cerians and Lambians are human species evolved from earlier species-Homo erectus that was abducted from Earth, Homo Erectus on Earth evolved into two different species Neanderthals and Homo Sapiens.
After the war Cerians mixed with Homo Sapiens because they were similiar creating modern looking Homo Sapiens and they killed Neanderthals because of their similiarities to Lambians. Lambians just relocated on another planet so they are the same species so Jevlenese are Lambians that currently live on Jevlen.
At the time the story takes place there are two Human species and and in word "subhuman" the "sub" means under or below.
It is not like "under a person", but like in "all species under the name of Human" , so while term "subhuman" is not the best it is better than demihuman.
Well I probably wrote too much but this is what I think about this issue. Correct me if I'm wrong.