The ICIS manual talks about the Kilrathi tendency to deify/demonize any enemy left alive (a process occuring with Confed at the time)...which could be taken as the Mantu being a peaceful yet superior race to the Kilrathi.
The Nephilim probably ARE the Star-Gods of Kilrathi prophecy, but they aren't the Mantu...unfortunately, they seem to be mostly aliens with delusions of grandeur. In all likelihood, they came upon this section of the galaxy as it was developing, wanted a conflict, and picked the race which showed the most promise in this direction: the Kilrathi. They set Kilrathi social evolution into motion, then probably returned to their section of the galaxy and kept moving coreward.
The Mantu were probably a peaceful race of explorers who happened to have fairly advanced weaponry (much like the Federation or Confed). They fought the more aggressive Kilrathi to a standstill, and probably just left the system after that.
I don't think the ICIS Intel report totally discounts the Mantu as being war-like...it just says they probably weren't. I don't have it sitting next to me right now, so would anyone care to prove me wrong?
EDIT: Just because the Kilrathi call them Gods doesn't mean they are...walk up to me on the street and ask me if I'm a god. I'll say, "Yes.", just because I learned a valuable lesson from Ghostbusters. It doesn't mean I, or they, actually ARE gods..."Advanced technology is often indistinguishable from magic,". Just because the Kilrathi are gullible cats doesn't mean that the Nephilim are gods, they're just an alien race with too much bluster. And, their advances in the intervening millennia were probably much less then ours. So, in short, we're the top gods around this section of space.
[Edited by Nep Parth on 05-17-2001 at 14:23]