In-character is super important since Genthru loves to underestimate opponents. When he sees that Kira doesn't even use Nen, he'll think he won't need it and just resort to purely physical means like he did with Gon.
Killer Queen turns the copycat into a bomb because he's an idiot who enjoys giving people the chance to fight back. He can't see Killer Queen unless he uses Ten? to detect the Stand, if that's allowed, of course.
But nah, Genthru isn't allowed to see Killer Queen since he already has a very easy wincon because all he has to do is flex and Kira goes batshit insane. If he can see KQ, he'll probably assume it's a Nen ability and go straight for Nen crush.