The display matrix is sandwiched between two sheets of glass. If you did truly read the article the other poster linked to, you'd find the sentence:
"The MacBook Pro with Retina Display uses a similar LCD construction to the MacBook Air/matte-MBP, without a cover glass.
Instead the Retina Display's two glass layers are different sizes, which you can sort of see in the cross section below:..."
That would clearly make the display's surface itself glass. You even said so yourself.
Seeing as on the hardness scale most plastics are much softer than glass, there is no way your keys would be able to scratch the screen regardless. Unless, of course, something hard such as a grain of sand or a metal shaving is stuck on
top of them.
Edit: And from Apple's very own website: "The Retina display doesnt sit behind a layer of glass, it is the glass"
http://www.apple.com/macbook-pro/design-retina/
I was merely attempting to save you money on a protector film which does nothing but hinder image quality, seeing as my own anecdotal experience owning 2 MBP with a similar construction since 2008 and seeing no screen damage from my keyboard's keys tells me such a film is unneeded.