There's a suggestion at the end (well, they did it, anyway) to apply box-sizing: border-box selectively, only where needed. That was probably smart and worked for these devs.

Don't do this in your codebase. This is maybe the most toxic and chaotic CSS change you can make to a codebase. Some devs will realize it's a box model thing right away. But the others will suffer in silence and confusion for quite a while. Both groups will hate you for this practice.