@Emilia42 Oh, I get it. I couldn't imagine there was anything stopping them from writing in and asking to book, though.
But here in this screenshot I'm in an incognito window, not logged in to Airbnb, therefore with no account, and it says "Request to Book."
When I click that I get three options: Google, Facebook, or sign up with Airbnb.
Choosing to log in with Google, I was then given a screen to create an Airbnb account.
I didn't want to go farther, so I stopped before I could find out if you'd be required to verify ID or upload a photo in the next screen.