This is explained in the post. The reading time is scheduled for everyone as part of the meeting time; therefore everyone is guaranteed to have time reserved to do the reading. If you only do the document, or only schedule the discussion time and not the reading time, some people will not read the document. I can certainly confirm that from 30 years of experience in companies large and small.

I’ve never worked at Amazon, but it sounds a bit off to imply it isn’t a results-oriented culture.