There are implementations of cp out in the wild that do not recognise the --recursive flag. OpenBSD was mentioned in the article and there’s also busybox cp https://busybox.net/downloads/BusyBox.html

Double dash long options are basically a GNU extension. BSD utilities generally don't support them. Apparently macOS does not either (since it's based off of FreeBSD)

I believe that -R is the safe works-as-expected-everywhere option.