It looks like you have to explicitly select the “Search in this topic” option. I just ran through the tutorial, and when it got to that point, I searched for “capybara”:
…and pressed Enter:
However, if I return to the search and click on the “capybara in this topic” option:
…it works!
The tutorial does say (emphasis mine):
Try searching for the term “capybara” in this topic
…but I agree that it’s not obvious. I suspect the tutorial was written while Discourse still supported “search as you type”, so the search result would appear before you even pressed return. (Also, I believe “search as you type” also searched private messages).