The promo was aired on television so it's not considered a spoiler.
It could be a hallucination since Kane did get beat around quite a bit but I don't think it is, I think he's actually there. And I agree with you on how dumb it would be to have Jaha survive and make it to the ground only to kill him off before the mid-season finale. I could see them both refusing to pick up the knife. Or maybe it's a test? I'm trying to be hopeful that they'll both be walking out of that grounder camp by the end of the episode.