I'd be guessing here, because he was headed east, that what he might have seen is Ground Speed.
Somewhat depends how high up in altitude you are flying, but he's talking about 12,000 ft ASL. Your IAS can be pretty low up there, compared to your TAS. Add in a decent tailwind, and your ground speed could easily be that high.
I've seen GS personally that high a couple of times, once heading East to PHX area for Copper State Fly In, in early Feb, and another time in August, flying to Henderson, NV. Both were 9500 ft elevation trips and both had a good 20 kt tailwind most of the trip.