Ultrasound researcher here: this work is almost certainly not full wave inversion. You're right that FWI can be done in this kind of setup. Notably, a lot of people are trying to use circular arrays of ultrasound sensors to do FWI, particularly in the brain or the breast. But, it's a challenging inverse problem to solve and as far as I know, it's still extremely slow - nowhere near realtime.

The data they show is almost certainly a normal beamforming algorithm like delay and sum, possibly with some simple speed of sound correction. The most similar paper I know of is here in Nature Biomedical Engineering from a team at Caltech. https://www.nature.com/articles/s41551-026-01660-4