Because in the video the shortcut is working only on an iPhone. On macOS this shortcut throws off two errors, both obscure. First, it says “Safari Reader is not available”, even if I have Safari in Reader mode. Then it says “The requested operation is not supported”, but doesn’t show or explain what operation.
I think you’d be better off writing your own shortcut explicitly for macOS. Or, put an article in Reader mode on Safari, select all or part of the text, and use Share > Notes from the contextual menu. Does the trick.