That's the point...why is Australia so hell bent to denying entry to this one guy?
Honestly, it's partly self inflicted.
One of the biggest news stories in Australia over the last couple of weeks was whether Djokovic was going to be able to play. I'm not saying it was front page news everyday, but it was in the newspaper virtually every day and constantly took up at least a minute or so of TV news most night.
It had been made very clear by the Australian government that he needed to be double vaccinated.
Suddenly, he posted something on the net - on Instagram, I believe - saying he had a vaccine exemption and was about to get on a plane to Australia. This was, I believe, quite late in the day here after normal business hours - but I haven't actually checked the exact timeline.
At this point, people in the Department of Home Affairs, and in Border Force saw the reports and their immediate reaction was - "Who gave this guy an exemption and why haven't we heard about it until now?" So while he was flying in overnight, people were trying to figure out how he'd got an exemption and nobody could find anybody who had issued one. So Border Force was ready, when he landed, to carefully examine the situation because they'd checked, having carefully rechecked what the rules were, and been told "As far as we know, he doesn't have one."
So they checked - and he didn't have a valid one.
If he had simply got on the plane without any publicity, he would have landed in Melbourne and walked up to Border Force. They probably would have asked him for evidence of an exemption, he would have handed over his letters from his Doctor, and they'd have read "Medical Exemption" seen it was signed by a Doctor, and just waved him through. It might have turned into a story, but I doubt it.