Does an avocado seed help guacamole stay green?
You're correct - it's a myth, as is adding something acidic to it like lemon juice--see explanation here. As you said, what causes it to brown is the oxidation, and that's just exposure to the air. If you wrap a cut avocado (or guacamole) in plastic wrap so there's no air space between the wrap and the avocado, then it will stay green longer than areas where air is getting to it.