It concentrates the sun from what I could understand from the article. Should work the same for any spherical bowl (parabolic would work even better) that is shiny enough.
The curvature of the bowl would also influence the "focus point", apparently Blanda is curved in such a way that the mirror's focus point is within the bowl, thus being able to set its contents on fire (in the same way the walkie talkie is curved such that its focus point is the focus point is on the sidewalk opposite the building)