I’m going through B10.0 and agree with point 1 of the OP. The B10.0 book, at the end of p 137 says:
Start up the server, log in, and point your browser at /promo. You should see the following:
But trying to view the /promo path at this point will throw an error because the code that follows on pp 138 is needed for the page to load successfully.
It would be better to not ask the reader to load the /promo path until after the code on pp 138 otherwise (as I did) the will end up thinking that the error was due to a mistake in the code they wrote rather than the fact that the code isn’t complete for rendering the view.