It's more of a proposed explanation than an assumption, but I think there are better explanations.
Penitentes are formed by ablative erosion so the peak of the penitente is indicative of an older and higher surface level - the penitentes are not built up from the original surface but are what's left.after the surrounding material has been removed (by the aforesaid erosion). If we have 15m high penitentes then we need to know where that 15m of eroded material came from and where it subsequently went.
More likely, imo, is that the 'roughness' is due to either compression fractures, similar to what we see in the Arctic ice sheets, or the presence of cryovolcanic 'spines', similar in mechanism to those we see being erupted from lava domes. Neither of these explanations require the now missing eroded material.