This isn't actually a book about writing CSS, as in the stuff inside the curly braces. It's a book about the organising and architecture of CSS; the parts outside the braces. It's the considerations that can be happily ignored on smaller projects but actually become the most difficult part of writing CSS in larger projects.
Terms like CSS at scale, or Large-scale CSS can seem quite nebulous. I'll try and clarify.
When people talk about large scale CSS or writing CSS at scale there can be a few possible metrics that relate to the large or big part of the description:
It might be CSS that simply has a large file size. There's a lot of CSS output and so making changes to that codebase can be difficult, as there is so much of the code to consider.
The CSS could be said to be large due to the complexity of the user interface that is being built with it. The overall file size may be smaller than the first situation but there may be a great many pieces of user interface that's codified in those styles. Considering how to effect changes across all of those visuals may be problematic.
It might be large CSS simply due to the number of developers that have, are, and will be likely to touch and change the CSS codebase.
Or, it can be all the above.