  We argue that while the experiment of Remez et al. is interesting and its
conclusions may well be correct, neither the original paper nor the recent
comment by Karnieli et al.[arXiv:2102.13345] fully discusses and rules out all
alternative classical hypotheses that could have led to the same wide azimuthal
distributions, ascribed to Bohmian-like quantum nature of an electron packet.
On the contrary, there exists a family of classical effects that could also
have resulted in the measured distributions and that were neither discussed nor
even mentioned by the authors. Such alternative hypotheses include {\it (i)}
effects of the beam sizes, of its angular divergence, of the temporal coherence
of the radiation process, which is also related to how the wave zone is
defined, and {\it (ii)} influence of the grating shape and of its material --
the effects that are known to be of crucial importance for Smith-Purcell
radiation from non-relativistic electrons. In order to prove a hypothesis, all
its alternatives must be checked and proved to be inconsistent with the
measurements. In our opinion, this was not properly done in the discussed paper
and in the comment. Finally, we propose to repeat the experiment and to measure
diffraction radiation from a thin metallic semi-plane or a strip. In this case,
the aforementioned classical effects play a much smaller role and the results
of such measurements would have higher credibility.
