Received my response now. They adjusted the rate so that the service charge is included in my final rate so they basically lowered the daily rate and had a lower service charge on marriott.com. They can't remove the service charge completely but they can adjust the daily rate + the service charge = correct final rate.
My 3rd party was very clear that all fees were included in their rate so I took a screenshot and attached it in my response. Also wrote that all 3rd party hotel sites from the EU are forced to show a final rate including all fees. Think that made it easier for them to make their decision.