Of "upper bound speed limit" and "speed of light" to avoid confusion.

Speed "c" is special case of speed of light, where path of light is unimpeded by any material, hence "speed of light in vacuum".

Many people say that "speed of light" is upper limit to any possible velocity, and yet I can think of a case where "speed of light" is slower than rush-hour traffic...

Many educated individuals: physicists and the like, automatically take "c" to mean "local speed of light in vacuum", however, general public gets the terms "upper bound speed limit" and "speed of light" confused.

Even GR, a theory of spacetime and matter, says that "Speed of light" is not globaly constant... because gravitational fields can alter distant spacetime geometry to the point that global speed of light in vacuum is altered.

So nothing new here... except for new way of "obstructing" light's path via quantum fluctuations or whatever. Dispersion is dependent upon frequency, hence measured speed of light in supposed "false vacuum" would seem as if speed were affected in different amounts for different frequencies, since different frequencies respond more or less severely to different obstructions/obstacles.

A full theory of quantum gravity would have to account for quantum flucuations in spacetime, therefore perhaps they're catching glimpses of "gravitationaly induced dipersion" due to frothy, ill-defined, jittery metric of fluctuating quantum spacetime. This would make GR's aspect of "global" seem a little more "local" because afterall, we're all "distant" observers of even the seemingly closest events, even though intergalactic distance scales are FAR MORE vast than say feet or inches.
Because a phenomenon that happens right at arms-length in front of you would seem "local"... at the quantum scale, even two-feet is a VAST distance (somewhere around the dimesnion of 4.1 x 1034 Planck lengths; if a planck length were thought of as the "quantum version of a 'foot'"... then a human's "arms-length" would be 2.8 x 1035ft, a seemingly VAST distance indeed!.. enough to span the universe a whopping half a billion times!).