It's fine for you to think that a recent case of Covid "should" be an exemption, but the fact is that it is NOT currently, in Australia, as we now understand. If this was miscommunicated to him, that is unfortunate, and this will all become more clear, one hopes. But the laws and regulations are what they are, whether you agree with them, or not. The burden of adhering to them falls on the person visiting the country. Djokovic has a team of people to help him with these things, and given the precariousness of the situation, and importance of the event in tennis and legacy terms, I would argue that someone around him should have done more homework on his behalf. Like filing for the correct visa, and making sure that all of the paperwork was air-tight.