So many even reasonably advanced learning materials calling dictionary form ‘casual’

It's not, and beyond absolute beginner shouldn't be taught as such. Tobira Intermediate Japanese or something literally uses it for the whole textbook. Even in the names it shouldn't be encouraging wrong overly simplified information when dictionary form has multiple uses, including formal ones, and the difference between formal and casual and polite and plain languages should be taught properly, because there's not such a clear distinction in English, so textbooks shouldn't be blurring them.