In the tutorial, you connect to alpine1 (ONE!), do some stuff, can't ping alpine2 and I guess the author was confused at this point because immediately after not being able to ping alpine2 comes detaching from the one you're connected to and that's alpine1.
The explanation of dit in the "Communicate between a container and a swarm service" was helpful but wasn't written in an earlier walk through on the same page. I just simply added it to the "Use an overlay network for standalone containers" walkthrough so that people come across it in case they only see this one walkthrough.