  In a recent Letter by Romanenko et al., the authors used low-energy muon spin
rotation (LE-$\mu$SR) to measure the Meissner screening profile in cutouts from
Nb superconducting radio frequency (SRF) cavities, systematically comparing how
different surface treatments affect the screening properties of the elemental
type-II superconductor. They reported a "strong" modification to the character
of the screening profile upon mild baking at 120 $^{\circ}$C for 48 h, which
was interpreted as a depth-dependent carrier mean-free-path resulting from a
"gradient in vacancy concentration" near the surface. While this observation
led to speculation that this surface treatment yields an "effective"
superconducting bilayer, we suggest that its likeness to such is accidental and
that the behavior is an artifact from the analysis.
