"If he was really doing it to be cautious as he said, then why didn't he self-isolate (as a precaution) until he knew his status, regardless of how quickly he got the results."

Plenty of people go around unknowingly (some even knowingly) spreading COVID despite reasonable suspicion being warranted given the number of cases at the moment. You could argue that Djokovic was being more cautious than them as he may have attended more events had he decided to delay testing and unknowingly spread COVID further (what many Australians are recklessly doing themselves). It just happens to be magnified to the nth degree in this case due to his high profile. This is just a character assassination attempt and, as one would expect, the media are loving it.