Hi Tim,
Indeed, the syntax of tess_smooth_sources changed and is documented in the very code of the function:
W = tess_smooth_sources(Vertices, Faces, VertConn=[], FWHM=0.010, Method=‘average’)
This function returns a smoothing kernel W, which is a linear operator [Nsources x Nsources] that you apply on your source matrix to smooth them over the surface. It does not estimate anything.
Cheers,
Francois