According to a few different bathymetry maps, the depth in that area of the South Atlantic could be up to 5km deep, unless it mostly occurred from shallower ridge areas around the islands. But the size of the anomaly on VentuSky suggests to me it was in a deeper part of the ocean. If the deep ocean has warmed that much, then this could just be the beginning of these sorts of events.
The "shallow" and "subsurface" in the above phrase "shallow subsurface methane hydrates" refers, I believe, to not far below the surface of the seabed, rather than shallow water depth. In fact, it seems that sediments accumulating at shallow water depths (<200 m) cannot contribute to gas hydrate accumulation since marine methane hydrates are only formed at significantly higher pressures, i.e in sediment deep under water.