A simpler way to look at that would be:
If you're sitting at the gate, you must cunsult the MEL for your problem, as it's a problem before the flight starts. Whereas, while you're taxiing out, any problem you have now will be considered by maintenance as an "inbound" snag at the next base (unless you don't go). But you'd be velly velly silly not to look at the MEL anyway.