Source?
Seems odd to launch tactical ballistic missiles for a demonstration, as a genuine tactical ballistic missile would pretty much fly out of visual range of anyone observing from near the launch site.
Everything I have seen says it was 300 guns. Basically howitzers, which are TECHNICALLY ballistic weapons.
I haven't seen anything that even implies that any missiles were fired in the exercise/celebration. I would be surprised if they even used a short range ballistic missile for the celebration. Even if the launch itself were impressive, they are not cheap pieces of equipment. It would be foolish to use them as a party favor. On the other hand, their ballistic missile tech isn't terribly advanced. They could have been the equivalent of like V2s. Pretty cheap to manufacture. They wouldn't need to be able to hit anything in this kind of display. Heck, as long as it made it off the launcher, as far as the people would be concerned, it was successful.
The other thing they reported, which amused me, was that they used aircraft to fly "zero feet above sea" and drop bombs on simulated targets. Why do I find this funny? Well, aside from teh fact that zero feet over sea I don't believe is used as a phrase by most countries. It did used to be used to describe minimum altitude runs, though they weren't used to actually drop bombs. That was how you GOT to your target. Second though, the planes they used were MiG23 and Su25. Neither of these planes is safe to fly nap of the earth, even the ones used by Russia, which has actually been able to upgrade their avionics. The chances that North Korea has avionics good enough to do it safely, or that their pilots have the necessary skill to do that, makes me laugh. Granted, maybe the pilots were some of their elite pilots.