The modern mobile phone has a barometer, GPS, accelerometers, microphones, timer, radio links, a huge amount of computing power, and a massive support ecosystem of development tools and optional hardware. It's also an everyday item and possession is entirely unremarkable. You can develop the software anywhere in the world and deploy it anywhere else on demand.
You couldn't hope for a better platform to design an efficient and flexible bomb trigger mechanism, all for a couple of hundred dollars and accessible to people with hobbyist-level skills.
That engineering problem has been solved.
I don't know, of course, whether this is what was used; a cheap wristwatch is still a sensible alternative. But this is what I suspect.