Not a physicist, but a cyclist and an engineer--
If the population travels as 'c' on average, and they have proven that some photons slow down... Doesn't that mean other photons MUST be traveling faster than c? My impression is the relativity has no bearing here--by traveling at 'c' they are already breaking that equation. The peloton works because some move back while others move up. This blurb seems to only discuss the "back" part.
Try reading about phase and group velocities. In fact some EM waves have velocities above c, but these can't convey information so aren't a problem for relativity. This article has a decent discussion of it and other things that go faster-than-light.