Gary
This link has quite a lot of research data from the Phoenix islands including Nikumaroro. The slope on the reef is given as being between 45 and 85 degrees depending on how far you are from the surf line and the location.
http://www.phoenixislands.org/pdf/00589.02.pdf
That report is interesting but its purpose was to catalog the reef area for ecological research purposes. It does make the general statement that the slopes vary from 45° to 85° but that is not specific to Gardner. In fact it points out that offshore of Gardner "on the western shore where the reef drops from the intertidal rim to > 40 m within 50 m
in some locations." The clear reading of this is that most of the reef is not that steep but only in
some locations is it that steep. Taking "> 40 meters " as meaning approximately 50 meters, within 50 meters makes the slope,
in some locations only, approximately 45°. This is apparently so unusual that it required this specific comment.
Looking at the bathymetric map again and this time looking to the east along the 4° 40' S parallel and looking at the spacing between 1,000 and 2,000 meters of depth covers 1.9 NM, 3,600 meters making the
deep slope in that area only 17° so the 45° to 85° slope statement does not appear to be applicable to the reef around Gardner.
So unless there is some reason to distrust the bathymetric chart I think it trumps the general statements in the report you referenced.

gl