No, it was never proven, but that's kind of my point -- that the speculation that the beings they were killing might be sentient was so random and unnecessary to the story that it felt like a gratuitous bit of nastiness. And the fact that the characters reacted as though the possibility of their sentience made no difference. They didn't debate the morality of killing possibly sentient beings and convince themselves that it was a grim necessity; they just plain didn't give a damn. I'm not talking about whether it was actually true; I'm talking about the sheer callousness of the characters toward the very possibility, and the sheer poor taste of injecting such a disturbing notion into the story when it served no imaginable purpose one way or the other.