why does the base color matter at all? What is “base” anyway when every word has a syntactic meaning?

Well.. the base color is about establishing a baseline of neutrality so that the deviations (the highlights) actually register as signals. Like he said "if everything is highlighted, nothing stands out". If you highlight an entire page of a book, you haven't highlighted anything, you’ve just printed the book on yellow paper.

why does the base color matter at all? What is “base” anyway when every word has a syntactic meaning?

I think there exists both passive usage of colors (feeling the structure through colors) and active usage (consciously looking for "green" when you need a "string"). The author is suggesting that with too much highlighting you can't use the latter.

But the best part is that the post contradicts itself: the suggested minimal theme doesn’t even address that typo use case mentioned above, because it doesn’t feature a distinct color for special keywords. So if one were to follow the post’s advice, return and retunr would look exactly the same, making it worse than the colorful theme it criticizes.

True, but I think he showed that to illustrate a broader point that current themes are so noisy that even when color changes you don't notice it, not that somehow his minimal theme would help spot it.