Idk about 20% more penetration. Any material the light goes through diminishes it somewhat, but I doubt the glass on a panel could account for that much. That's just my opinion though. Where did you find the info?
That being said, dust does eventually build up on the inside of the glass, and after a few months you can see it. It's not a whole lot, so I just take the light apart gently and clean the inside of the window. Takes like 10 minutes. It may void the warranty, I'm not even sure about that, but it's easy and they're built well enough to hold up to disassembly of the case.
With how many times I've accidentally thumped my cranium against my light, I'm glad it's got the glass.