...whether you could beam over...
Well, David Marcus clearly says "You can't".
I always took "You can't" as "You can't stop it
According to the transcript, that's exactly what it means. "You can't stop the detonation," not "You can't beam over."
SPOCK: Admiral. Scanning an energy source on Reliant. A pattern I've never seen before.
DAVID: It's the Genesis Wave!
DAVID: They're on a build up to detonation!
KIRK: How soon.
DAVID: We encoded four minutes.
KIRK: We'll beam aboard and stop it.
DAVID: You can't!