I don’t think they sound much like flats but maybe that comes later?
As is they do sound pretty good, the tension is just right and I was surprised by how good the B string is
Compared to LaBella DTF's you are correct. They have much more sustain than traditional flats.
IMO they sound more like flats when you use a pick. Great for classic rock tunes (especially ABB tunes). They really sound great on a fretless too!