The police confirmed that there were working, homemade explosives at his residence. Explosives are not difficult to make. So yes, it does follow that having no gun would have meant he used the explosives.
Is there a guarantee that all of the explosives would have gone off? No. But then, his 100 round magazine equiped "military grade assault rifle" only killed 12. So 88% of the bullets didn't kill anyone either. But a dozen bombs would have killed far more.
If they worked, if he could have deployed them. That was the point. Bombs are actually kind of difficult to deploy and even when successfully used, they don't really do that much damage.
The Boston Marathon bomb was set off in a crowd and only killed three people.
This guy was able to kill 12, and I have to assume he didn't have much training on how to shoot.