The catch is that the page you reference is about JQ International connections to UA ... helpful if you were flying JQ to Bali or Singapore from MEL but not applicable to ADL unfortunately. So you will have to carry your bags to the JQ counter, meet their very strict cutoff time for baggage check-in, and argue about the entitled allowance. JQ is an LCC, so charging for baggage is a big part of their revenue. UA should have booked you on QF which would avoid this problem as QF will accept the IATA allowances. Presumably UA got a cheaper price from JQ than for QF MEL-ADL