The Figure 3 label includes: Note the large areas where depths have not been measured from ships. (ref my previous post)
There is not any claim of detailed accuracy, but the graphics make it obvious that the Geoid undulations on the surface cannot possibly indicate sharp anomalies in the sea floor such as narrow, deep canyons or relatively small (but significantly deeper basins.
After all, these features are NEVER observes on the surface.
Here is another page on satellite imaging of the ocean.
Please note the Prediction of Seafloor Depth section which explains that satellite altimeter measurements (converted to depth measurements) are not very accurate and are not used to assess navigational hazards but are used in combination with sparse (surface) measurements of seafloor depth to construct a uniform resolution map of the seafloor topography.
Yours Sincerely for Accurate and Informed posting . . . . . .
That was the whole point-there are other ways of mapping that DON’T rely on ships.