  We argue that the elastic backbone (EB) (union of shortest paths) on a
cylindrical system, recently studied by Sampaio Filho et al. [Phys. Rev. Lett.
120, 175701 (2018)], is in fact the backbone of two-dimensional directed
percolation (DP). We simulate the EB on the same system as considered by these
authors, and also study the DP backbone directly using an algorithm that allows
backbones to be generated in a completely periodic manner. We find that both
the EB in the bulk and the DP backbone have a fractal dimension of $d_{b} =
d_{B,\rm DP} = 1.681\,02(15)$ at the identical critical point $p_{c,\rm{DP}}
\approx 0.705\,485\,22$. We also measure the fractal dimension at the edge of
the EB system and for the full DP clusters, and find $d_e = d_{\rm DP} = 1.840
\, 54 (4)$. We argue that those two fractal dimensions follow from the DP
exponents as $d_{B,\rm DP} = 2-2\beta/\nu_\parallel = 1.681 \, 07 2 (12) $ and
$d_{\rm DP} = 2-\beta/\nu_\parallel = 1.840\, 536 (6)$. Our fractal dimensions
differ from the value 1.750(3) found by Sampaio Filho et al., whose value may
represent a crossover effect.
