You're right to mention it because in the 2nd step "Centering", I use the same technique: adding a max-width and using auto margins for the left and right sides.
On a side note, this reminds of the "Doing it more vs. doing it better" thread that was here on HN yesterday , and how the JS code of "Web Design in 4 minutes" is quite bad… I just wrote those 50 lines directly in the HTML, at the end of the page, and tested it manually a few times. As a result, it is tightly coupled to the markup, and still has lots of bugs. But I imagine that if I had focused on writing "beautiful" code instead, I probably wouldn't have shipped the project in the first place.
Last year I started to build a web app without knowing how to code -- I started not knowing the difference between CSS and JS. You can imagine how confusing the current ecosystem is to beginners. (My co-founder and I taught ourselves to be the technical co-founders). Your tutorial really helped, and I'm using Bulma :).