Not really
According to radio-locator Mankato is just outside of the fringe area
I personally accept a fairly loose definition of Tropo:
First of all, I presume you do not usually recieve this station, and then you had it come in like a local.
Another test I can perform is if you notice several other stations from the Mason City area, and/or you can confirm that KBDC hasn't changed their pattern in any way.
The best time to recieve Tropo is overnight or in the early morning. You said "this morning", so you're right in the zone.
Finally, there's a difference between the tropo duct and a troposhperic enhancement. A duct is the classical definition of Tropo, where signals will air-duct to your town or city from 300+ miles away, whilst enhancement (what you experienced) takes signals from just beyond the horizon, and slides them into your area, often being very strong, like your station.
You may call it what you want, however I think you have Tropo, just TE instead of a Duct.