Okay, I don't want to be obstinate about this, and I'll grant that your interpretation could
be correct. But I still submit that the dialogue you've provided doesn't indicate that without the possibility for another interpretation.
In the first conversation, McCoy is rightly discussing the fact that while Genesis is meant to create life, it will also destroy any existing life it encounters. But that is in relation to it being fired at a planet and detonating upon impact with the planet, which it was designed to do. Kirk and Company have no idea what the Genesis device would do if detonated in space or aboard a starship, because it wasn't designed for that purpose. Maybe it would destroy all life within thousands of kilometers. Or maybe it would just fizzle out and the wave have no effect at all if it doesn't impact with a planet.
In the second conversation, obviously, the fact that Genesis is about to detonate is a point of major concern. But it still doesn't say that it's a point of major concern because the Genesis wave will impact the ship and destroy everything. It could just be that the Genesis wave will detonate the antimatter reactor aboard the Reliant and the resultant explosion will destroy anything that's too close.
And, while we're on the subject, why does Genesis even have a "countdown" detonator anyway? We see from Carol's video presentation that it is meant to be fired at a planet and detonate on impact. There shouldn't ever be any reason to need a countdown.