Kirk says we'll beam aboard to stop it which David says can't be done.
...Which makes one wonder exactly what
cannot be done?
Shut down the device?
Or beam over to attempt anything in the first place?
David might simply be saying that transporters won't work anywhere near an activated Genesis Device. Any attempt at beaming a shutdown team aboard, beaming out the device, or destroying it or putting it in stasis with transporter, are thus categorically doomed to fail.