This is common. They are indeed referring to a MAPD
I have clients all the time say, "my doctor wont take UHC, Aetna, MoO" etc. I let them know it's not their choice. I let them know that their plan is not primary, Medicare is and if the doctor accepts Medicare then they can see them. The doctor bills Medicare. End of story. I let them know that what they are referring to is an Advantage plan, which would be primary to Medicare and then they would have to make sure the Dr. Takes it.
There is no contract with a Med supp and a Dr. If that doesn't work, I tell my client that we can give the office a call together and I will explain it to the lady at the front desk