> mixed with basic rules about subject-verb agreement (...) [that] I would expect as a basic requirement for passing high school English

I reckon this is just a poorly picked example on your end, because the guide explicitly states the following about that:

> There are two forms of agreement: subject-verb agreement and pronoun-antecedent agreement. Subject-verb agreement is pretty rudimentary, and is not discussed here.

Regardless, sometimes (oftentimes?) technical documents are written by people who are not actually technical writers. A good number of those will also have a native language other than English. And in a lot of high schools, passing the English class is really not a very high bar, especially when failing people en masse is not really an option. You can only coerce people to learn a language so well.