[…] with WordPress child themes, you are all but guaranteed to get into the weeds of specificity, hunting around theme stylesheets that you didn’t author, trying to figure out what existing declaration is preventing you from applying a new style, and then figuring out the least specificity you need to override it, and then thinking “Maybe it would be faster if I just wrote all of this myself”.
Her point wasn’t child themes (although I think that’s a perfect thing to point to as the way you work with them is all with overriding what is already there), but the expectation of knowledge:
[…] unless you are “a CSS person” this understanding of specificity and its impact on the future of the code-base is somewhat specialized knowledge. Should everyone who writes CSS be expected to understand these details? Maybe, but the more experienced I become in all kinds of development, I’m starting to think that’s an unrealistic expectation given how much other stuff we have to know as developers.