It reverses Unicode code points

No. It doesn't know anything about Unicode and there's no requirement for the string to be Unicode text.

not characters in the usual, well-understood sense of the word.

In my experience, "character" is the constituent element of a string, and never a grapheme except by happenstance. Let's just say there is no such consensus. Regardless, that's the definition used here.

If I understand the design principles of Perl correctly, the reverse function should properly reverse extended grapheme clusters

No. reverse provides a vital string operation. It should not assume the string is Unicode text. Reversing text is also a useful function, but it is not provided by reverse.

Same goes with substr and length.

when the thing being reversed is Unicode text (and Perl understands it is Unicode text), and it should reverse bytes otherwise.

Perl doesn't have a means of "understanding a string is Unicode text".