I have a website I run that is focused on sportsbetting so while I know high level what is legal where, I'm not actually in Minnesota, so I don't know every single detail especially when it comes to what's allowed in person.
My guess would be that either A) that's tribal land and there's an exemption for that in the current laws or B) they have a casino, but don't offer sports betting or C) they only allow sports betting on the property by either requiring all bets to be placed in person or the app they offer is for sportsbetting is geo-fenced to their property (which is how states like WA do it)
Long story short it is a compromise between the Mdewakanton Sioux, the state, and Canterbury to keep gambling exclusive to Mystic Lake Casino and Caterbury. The State flirted with the idea of expanding gambling and started moving in that direction, but Mystic basically now props up Canterbury, lets them have some amount of gambling on site, and the State did not expand gambling and generally is protective of Mystic. It's like a pressure release valve on gambling with some small kickbacks to the nearby legacy horse racing track. It's a strange local arrangement. Grew up in Prior Lake and had a few tribe members in school so this is just my bias on the arrangement.
My guess would be that either A) that's tribal land and there's an exemption for that in the current laws or B) they have a casino, but don't offer sports betting or C) they only allow sports betting on the property by either requiring all bets to be placed in person or the app they offer is for sportsbetting is geo-fenced to their property (which is how states like WA do it)