  A recent publication on arXiv:1905.00668 suggests that the phase relationship
within a frequency comb can be reconstructed from a heterodyne measurement
using a reference comb. In principle, following this approach, such retrieval
of optical phases appears possible, but should give rise to a temporally
constant signal. Instead, arXiv:1905.00668 provides experimental evidence for a
pronounced relative drift of the phases, which indicates that the modes of the
comb are not coherently phase-locked. This temporal drift can be explained if
the comb under test is not equidistant. This further means that the underlying
temporal waveform is not repetitive and cannot be compressed into a coherent
short pulse. This artifact severely hampers suggested applications of quantum
cascade lasers in precision metrology. Moreover, the notion of designating
multi-mode quantum cascade lasers as frequency combs appears questionable.
