A couple of thoughts:
Determining if a tire failed due to a road hazard is not something an amateur can do very well. Even after 20 years of doing it, I occasionally screw up! I have found that I am much more successful if I can examine the inside of the tire. Trying to find a cut or puncture from the outside is pretty difficult.
"The outer side wall had completely separated from the tire right at the edge of the tread." This is a classic run flat condition. That leads me to believe this was a puncture related failure. Plus the mention of an sudden explosion further leads me to believe in a road hazard.
Innertubes? Yes that is puzzling. It's quite possible that the tube failed. I have found that valve holes for tubeless wheels are not mounted in the best location for tubes.
I don't like the excuse that if the tire doesn't seal, they put in a tube. Tubes are relatively expensive compared to what fixing the wheel seal would cost.
On the other hand, there are a lot of wackos working in tire shops.
Based on that experience I wouldn't give those guys any more business.