You know, an expert is someone who's knowledge proves to be practically useful and stands up to experience.
Any Glock expert should tell you that a Glock is capable of firing out of battery because it can. I just did it.
I took a fired 9mm casing, seated a Winchester Small Pistol Primer to the correct depth and loaded it into the chamber of a second generation Glock 19 serial #HB0xx US. Wearing eye and ear protection, I held the slide back 1/8" from full battery and pulled the trigger. It went off.
Afterward, I had trouble extracting. This turned out to be due to the primer bulging out of it's pocket and into the firing pin hole, preventing it from unlocking. No doubt this was caused by excessive headspace upon firing. The extractor kept the casing close enough to fire the primer but loose enough for the primer to expand back.
To confirm there was no problem with my 19, I did the pencil test with my brother's 3rd gen. 19 and 26. Both nocked the pencil out of the barrel starting from 1/8" out of battery.
So, if a Glock happens to not be in full battery, has live ammunition and the case is even less supported than normal due to excessive headspace, what's going to happen?
I say the slide will unlock from the barrel early and the pressure will go somewhere, probably through the most unsupported part of the case.
Am I worried? No. I only own a 9mm Glock, whose feed reliability with that slim, tapered cartridge is legendary.
I'm not predicting every .40 Glock will blow up. But a fair number have while firing this blocky, compromise cartridge. There are grounded, scientific reasons some fear .40 Glock pistols.