None of us know the full details yet, but I don't think it's a simple as that. He didn't need to provide proof he was vaccinated, just proof that he was exempt from vaccination. One of those exemptions is having had Covid in the last 6 months. He received exemption to play in the tournament on (presumably) that basis and then applied for a visa. Turns out this was the wrong type of visa and that receiving an exemption to play in a tournament is not the same as receiving an exemption to enter the country.
I do agree though that Djokovic is an absolute bellend AND that Australia could have handled this situation much better.