I'd say mostly BS, unless the bomber is a technical idiot. If any random cellphone freq can set it off, it could be set off before the bomber drops it.

Being detonated by cell phone, maybe setting all numbers but one to a ringer of silent and then using the speaker would seem more plausible, but figuring that out probably takes more time than they can spare right now.

sounds like a corruption of the (mistaken) idea that they shut off cell service to prevent the bomber from dialing other devices to set them off

Science behind what?

Can random usage of a nearby phone trigger remotely detonated bombs?

If the bombmaker wants random cellphone usage to trigger it, yes.