Keyboard protectors: do they prevent ventilation?

I'm a salesperson/service advisor through an Apple Premium Reseller, so it's a mildly dumb question for me to ask.

I had my 2017 MacBook Pro nTB out at work, with my Speck Smartshell and Moshi Clearguard on. This is the first computer I've put a keyboard protector on. I like this Moshi one; very thin, very responsive, well fitting.

One of my technicians scolded me (jokingly) for having the keyboard protector on. He then told me to take it off: apparently it can actually cause liquid damage on its own. Apparently it doesn't allow the computer to cool properly, and he has seen condensation build underneath the protector, which has fried several machines.

Of course now I'm panicked. I love the keyboard protector for keeping dog hairs and skin oils off my keyboard. Makes me feel a little more secure in regards to an accidental spill too.

Does anyone have any experience with this condensation issue and have some words of wisdom?

+ Is it true that keyboard protectors can cause screen damage?