I downloaded a county shapefile from the MnDOT Geodetics website in hopes of getting the WKT projection directly from the source, but the shapefile came without a .prj file. Foiled!
I would have thought that they may have changed the false easting value, but a cursory glance at the coordinates in the county file makes it seem like they are indeed just EPSG:26915. I overlaid a NAD83 county shapefile I had on hand and projected it into UTM15N for a quick check:
I don't have your specific data, so I can't say for sure, but this seems to corroborate Kevin's answer that it's probably just a projection naming issue. Their "UTM15 Extended" probably just means some counties or data falls outside of the UTM 15 border, so they "extend" the zone to encompass the entire state. None of the parameters seem changed, though.
You might even try removing the projection information altogether, since that seems to be how they are delivering data to users!