Super cool. In both sound and musical capabilities, it seems like it shares a lot with the viola da gamba, especially as they both compare to modern stringed instruments; gambas, being fretted, can't achieve vibrato in the same way that modern strings can, and so depend a lot more on ornaments (trills, mordents, etc.) for emphasis. They also lack dynamic range as compared to modern strings because of their simpler bows, meaning dynamics were often achieved by adding or removing voices in ensembles. This new instrument is similar: vibrato and dynamics probably aren't easily achievable, so it seems well-suited to gamba repertoire.

No surprise, then, that in the Polish-language video linked elsewhere in the comments here, he plays a couple of M. de Sainte Colombe pieces, as he was a master gamba performer and composer.