Often times, I think with these types of comments, it's less of an actual measurable "roll off" as it's more a perceived "softening of the edges" where things are generally less "detailed" sounding compared to other caps, in much the same way that Jupiter's copper foil caps are sometimes described as "warm," "dark" or "veiled," even if frequency-wise they generally measure identically to other capacitors of the same value.
It could be another factor, like discharge rate, caused by the dielectric that gives them some of that tonality, where the higher frequencies seem to lose some of the finer details.
