You pay the screen replacement price if they only replace the screen. You’re paying the full 79 because you’re getting a replacement device by using Express replacement.

In that link you shared there’s a section that says “Express Replacement Service: we’ll send you a replacement device so you don’t have to wait for a repair3” so if you go down to footnote number three explains it. It’s also a link to https://support.apple.com/en-gb/iphone/repair/express-replacement that states this info, too.