I love the way that's been worded. It does not say that they can fly it with full passengers and their luggage, and cargo, and appropriate weather reserves. There is a nominated payload figure of about 16,000 kgs. Carriage of that amount, whether self loading or otherwise, is full payload. So it's going to be shared amongst the passengers, their luggage, and any payload in the hold. And any time-limited cargo will bump passengers and/or luggage. This is a normal sort of calculation, but, I don't see this aircraft as being easily capable of the trip on any days with extensive holding on the forecast. The rule for Singapore that I used, was to carry at least an hour of holding when nothing was officially required. Lots more when it was on the forecast. It was amazing how often that saved our bacon.
Neither are particularly affected by jet streams. The issue is one of alternates, and the need for them. Singapore, and Darwin, both have horrendous weather forecasts as a normal event. Melbourne not so much.