I heard a rejection for an arrival into Sydney on 124.55 yesterday (I think - may have been 2 days ago). It may not be the same one as the OP mentions but if it was, the actual reason given was they hadn't booked an arrival slot at Sydney.
That is not an ATC requirement but a Sydney Airport requirement. (I think that is as much about the airport operators being able to invoice you as anything). I know of a multitude of GA flights into YSSY over the last few months (I have done a few myself). You have to book a slot to do it, it isn't hard. ATC have always been very accommodating. NB you cannot do a T&G. Must land and re-taxi.