I mean, it's called "LaTeX by example", so there's a pretty good chance it's written in LaTeX, which you do indeed compile to get the PDF or whatever output you want.

Also, just having access to the source doesn't make it open source - that requires more freedoms. For example, here's GitLab Enterprise Edition source code, fully functional and ready to be used. And also officially described as the proprietary edition of GitLab by the GitLab company itself. Why? Because its license pretty much boils down to "you can use this only for testing and development, unless you have paid for it".