Open top is probably fine for a lot of urban, day to day CCW use. Maybe it's ok for duty use too, but I'll defer to the LE folks.
However, I think they missed the most common way for gear to fall out of open-topped pouches - contact with another items, such as a door jambs, counter tops, tree limbs, brush, etc.
At a carbine class I can usually tell the guys who have never patrolled in rough country by the way they carry their mags. I've lost plenty of stuff from my beltline and LBE while patrolling (back in my ancient USMC days), or more recently while hunting in the National Forest and working in rough country as a field scientist. In those circumstances I'll go with closed top pouches every time. (Preferably dummy corded to the belt or LBE).
I'll also note that John Coreira from
Active Self Protection has found that reloads that actually influence the outcome of a gunfight are incredibly rare. His view has developed after watching 20,000 to 30,000 gunfights, so that's a decent dataset. (ETA: His dataset doesn't include .mil engagements. I'd posit that in .mil engagement reloads are more likely to occur, and have a higher degree of import.
YMMV