  In a recent paper by Husain et al. [PRX 9, 041062 (2019)], the two-particle
electronic excitations in Bi2Sr2CaCu2O8+x have been studied by Electron
Energy-Loss Spectroscopy in reflection (R-EELS) in the strange metal range
between underdoped and overdoped materials. The authors conclude that there are
no well defined plasmons. Rather they obtain a momentum-independent continuum
which they discuss in terms of holographic theories. In this Comment it is
pointed out that the experimental results are in stark contrast to previous
EELS in transmission (T-EELS), Resonant Inelastic X-ray Scattering (RIXS), and
optical studies. The differences can be probably explained by an inaccurate
momentum scale in the R-EELS experiments. Furthermore, it is shown, that many
material specific experimental results from T-EELS, R-EELS, RIXS, and optical
spectroscopy can be explained by a more traditional extended Lindhard model.
This model describes the energy, the width, and the dispersion of normal and
acoustic plasmons in cuprates, as well as the continuum. The latter is
explained by electron-hole excitations inside a lifetime broadened conduction
band. This continuum is directly related to the scattering rates of the charge
carriers, which in turn, by a feed back process, lead to the continuum.
