They used an autonomous underwater vehicle with upward-looking multibeam sonar (the main one seems to operate at 300-kHz) to map 140 km² as well as measuring ocean currents, temperature, and salinity at 20 to 80 m below the ice. Multibeam was new to me, Wikipedia says this "emits acoustic waves in a fan shape beneath its transceiver [using] beamforming to extract directional information from the returning soundwaves, producing a swathe of depth soundings from a single ping" (https://en.wikipedia.org/wiki/Multibeam_echosounder)

The researchers say that limited observational data on how the sea erodes this ice leads to uncertainty in sea level predictions and this new data should help future studies lower those error margins. I want to say cool stuff (no pun intended) but it's also frightening in a way... at least we'll better know what's coming for those who come after us

I don't see any mention of how close previous assumptions were to what this new information shows; probably this is not yet processed/applied enough to interpret it in that way?

The mission here was just imaging. I think the first ever imaging of the underside. So the estimates of mass with data on the underside are only just beginning