The rules have changed over the years, the SYD airport web site was correct in the past when QF allowed using the bus only if both arriving and departing on QF. They have broadened the rules to be any QF flight. Much more convenient and also much more logical. The real problem is that the privately owned for-profit airport wants to charge for transfer between terminals (as do the privately owned train station at the airport) and airlines had to work out the right to transfer their pax "for free". The gov't was a bit stupid in setting the contract terms and price when selling off the airports (perhaps deliberately, the buyers have made billions in profit and given millions in political party contributions). But the insane quest for profit at SYD leads to the wierd routing of pax to maximize their shop exposure and funny rules for things like the transfer buses.