I can't swear to the details but that is subtantially true (if not completely true). What happened is that TRAINS MAGAZINE used to publish an annual speed survey. And most years the Twin Zephyrs did come out the fastest. The survey did not measure top speeds, however, but rather, average speeds. The average between whatever two cities it was (whether the ones you mention or not I do not remember) was in the low 80 mph range, like 84.3 or something.
Obviously the top speed has to be well above the average, so 100 or so sounds about right.
The speed survey must have been a massive undertaking, at least the first year it was done. Beyond that, guess he (I think his name was something like Don Steffee) just had to look for changed schedules.
He did, as I say, publish average speeds, not top speeds, and it was between all the little towns, not end to end (as in Chicago-Twin Cites, for example).
It was a TRAINS tradition for many years. One year the City of New Orleans won the title between two certain cities in northern Illinois, but it ws usually won by the Zephyrs.
The Illinois Central published an entire brochure about it when the City won the title.