Here is a description of the allowable exemption claims. Unless there is something massive we don't know, I don't see what grounds Nole would have had for applying for any of them:
Exemption conditions
"Under the Australian Technical Advisory Group on Immunisation's (ATAGI) current guidelines, a medical exemption is granted to individuals who have an 'acute major medical condition (e.g. undergoing major surgery or hospital admission for a serious illness.'
The other remaining grounds for a medical exemption concern people who have suffered a 'serious adverse event attributed to a previous dose of a Covid-19 vaccine, without another cause identified' and a vaccinee who 'is a risk to themselves or others during the vaccination process,' due to an 'underlying developmental or mental health disorder.'
Lastly, exemptions may be given to anyone with a 'PCR-confirmed SARS-CoV-2 infection, where vaccination can be deferred until six months,' and in cases where individuals have received 'anti-SARS-CoV-2 monoclonal antibody or convalescent plasma therapy.'
In June 2020, Djokovic tested positive for coronavirus following an exhibition event he organized in Croatia, but since then there have been no reports of him being re-infected with the virus.
'It's ultimately up to him to discuss with the public his condition if he chooses to do that and the reason why he received his exemption,' Tiley said."---from CNN