  Contrary to the starting assumption of Grbi\'c et al. (Phys. Rev. B 83,
144508 (2011)), here we will first argue that a 16 T magnetic field is not
enough to quench all superconducting fluctuations above Tc in
YBa$_2$Cu$_3$O$_{7-\delta}$. We then conclude that through their measurements
of microwave absorption these authors actually determine the ac-fluctuation
magnetoconductivity at 16 T, instead of the zero-field ac paraconductivity as
they pretend. So, the temperature proposed by Grbi\'c et al. for the onset of
the superconducting fluctuations, T', will correspond to the one at which the
finite-field effects at 16 T become measurable in their experiments, and the
actual fluctuations onset will be located well above T'. These conclusions,
which also concern influential recent publications on that issue, are then
confirmed by analyzing some of the Grbi\'c et al., data on the grounds of the
Gaussian Ginzburg-Landau (GGL) approach for the finite-field (or Prange)
fluctuation regime.
