Steel belts are under the tread. The sidewalls are a different construction but it is possible that there may be a layer of material with some steel.
What's In a Tire | U.S. Tire Manufacturers Association

I would hazard a guess that you ran over something that tore into that sidewall. It could be a piece of metal that was at an angle so when you ran over the one side, the other side damaged the tire. It was some kind of road hazard that you didn't notice. (angle iron, bungee cord "S" bracket, etc.)

If it were me, if all of the tires are the same age I would swap the fronts and rears, staying in the same directional travel of rotation. Tires are pretty tough.

I know tire shops aren't allowed to patch sidewalls but another option is to put a new (make sure you watch them put a NEW) inner tube in the tire. I was driving around some new construction homes many years ago and got one of those carpet hold down nail things (big square metal head on a nail) in the sidewall of a brand new tire. The mechanic put in an old tube with a dozen patches on it and charged me for a new one. I put 50K+ miles on the tire without any issues, but it was on the back axle.