There has been no previous sea-floor surveying of vast areas of the Indian Ocean - and the small amount of sea-bed information previously available, came from satellite imagery.
I thought prior to conducting the search though they did a survey mapping the area. That link you gave says
Before the underwater search for MH370 could begin, it was necessary to accurately map the sea floor to ensure that the search is undertaken safely and effectively. Bathymetry survey vessels spent months at sea, scanning the sea floor with multibeam sonar to gather detailed, high-resolution data. The data has revealed many seabed features for the first time.
I figured that they would know about this volcano ?