That the only mapmaking authority to whom this island would actually matter is France (SHOM). Since it's in their territorial water and it also would extend their territorial water by being an actual Island. So it's in the very interest of France to have a clear idea what belongs to them and what not, also it's their responsibility to provide as accurate as feasible navigational charts to ensure the safety of ships which travel through their waters.

Look at the french charts featured at geogarage, the post has added a few more SHOM charts for reference which show no Island.

http://blog.geogarage.com/2012/11/south-pacific-sandy-island...

So what does it mean?

Basically one would only need to blame Australia for the lack of funding or interest to ground-truth and update their issued charts outside of their territorial waters. The easiest explanation would be a lack of communication between Australian and French nautical mapping authorities. Second to that the Australian authorities didn't feel the need to do their own mapping by means of actively acquiring of information on the area by satellite, plane or ship for civil applications.