27 mph over distance is conceivable for a reasonably strong amateur rider, especially in a group. Theoretically even if a rider is pulling the whole way, closely following riders offer the lead rider a bit of aero advantage.
At 60 with a hodgepodge of physical and health limitations I still have occasional good days when I'm stronger than usual. In the spring, before an injury accident, I managed 25 mph over a tough 6 mile roller coaster segment that ends in a sequence of climbs. That was good for 2nd on Strava for a couple of weeks. Then a friend snagged the KOM at 28 mph and bumped me down to 3rd. Then a couple of weeks ago three younger guys from a local fast club nudged me down to 6th, going 0.3 mph faster than my best time.
The difference is those guys could do that almost any day, any time. Ditto my friend who snagged the KOM. I can crack a top 10 only on an exceptionally good day, and it helps to have a tailwind or at least neutral wind conditions. All the KOMs and most of the top 10s in our area are wind assisted, so that alone doesn't nudge me into the top 10. I need to have an exceptionally good and increasingly rare day for that to happen, when the lungs are clear, no asthma symptoms, the legs are strong, the mind is sharp and I know the route well enough to pace myself perfectly.
So if an inconsistent old loafer like me can do that, the mythical 27 mph thing seems reasonable for serious and seriously fit riders, men and women alike.
Also, there are differences between KOM attempts, fast group rides and races. Phil Gaimon talks about this in his recent video narration of his informal race with Fabian Cancellara. They finished well behind the KOM, despite feeling like they were pretty fast. But the tactics and psychology of racing are very different from a solo time trial effort. And a cooperative small group acting as a team should be able to snag most KOMs, but in actual practice they don't often manage this for the same reason some pro teams don't dominate team TTs -- there's both craft and art to perfect team tactics and few groups manage to master it.