[publicfig]

This is still a typically stringed piano, just with felt added to dampen the other strings, I believe. The webpage describes an Una Corda piano built with only one string

[misthop]

One string per note (as opposed to the usual 1/2/3 depending on where in on the keyboard you are talking about), not only one string.

The video the parent linked to is the Una Corda - played by the designer/builder