Clipboard events are already supported by safari (which the page you've linked to notes) and they've got nothing to do with execCommand, they are about hooking and reacting to user-triggered clipboard operations e.g. the user pastes (via contextual menu or shortcut), the software can know about it and alter the content before it's pasted.
execCommand is about programmatically triggering clipboard events e.g. the user clicks a button in the page and the application translates that to a copy or paste event.