You are 100% correct. It is much easier to pull a trigger on a Glock or a Shield than a 12lb J Frame trigger. But in all honesty, it doesn't matter. A trigger is a trigger and when it gets pulled your gun is going to fire. That is why when you are holstering your gun, you do it very carefully. If your gun goes off when you are holstering it, regardless of the weight of the trigger pull, it is 100% your fault. There are no exceptions to this.
And when you holster a gun that you KNOW has a lighter trigger, you do it that much more cautiously. I know when I am carrying one of my guns with a light trigger, I put it in my holster extra carefully. I do it so carefully it is to the point I am neurotic about it. And that is a good thing.
I own over a dozen revolvers. All have trigger jobs. Some have competition trigger jobs and have a 4 to 5lb DA trigger pull, just like a Glock. You better believe I am watching those slide in my holster nice and safely every time they go in. And clearly this officer was not doing that. The gun is not to blame AT ALL. 100% his fault.