Despite the public perception that flights are still empty, they most certainly are not - though there are many fewer flights than in normal times. Getting 14 Government seats (and I would have thought that 14 is a very small party for what was supposedly an important visit) would be bloody hard without the airline bumping people off the flight. They will do that (well, BA will) for VIPs, but that bumps the price up very considerably as they will charge for the compensation, accommodation etc they have to pay those they bumped off. Also, it's normal for a fairly significant number of journalists to travel with the politicos, and their parent organisations pay for their seats on Government aircraft. It's possible there were none on this particular flight if it was at very short notice, but I suspect the story was broken by people who actually travelled with Ms Truss.
Just to add: getting 14 business-class seats return to Sydney at short notice, including the surcharge for bumping people off, would cost roughly double the regular J price, or in the order of £15-20k per seat. That's about the same as a short-notice First Class seat, but there normally aren't that many 1st seats on one flight even if there are no other passengers.