I have no idea on a metric of how frequently an “ordinary” gun jams, much less these modified ones, but I can apply some logic from my knowledge/experiences. The weapons you mention having experience with are designed with appropriate tolerances to not bind up under heavy use, so are a bit different from the ‘consumer-grade’ type we’re talking about in this specific event.

The type of semiautomatic rifles we’re talking about here use recoil to cycle the action. A bump stock allows the whole weapon to oscillate - and can have an effect similar to not securely shouldering the weapon. This prevents the needed energy from being transferred into the action for complete cycling, and that would make the weapon prone to jamming.

I don’t know if I have much of value to add to or reply to your second paragraph, but yeah that fixation is weird.