To some extent that is correct, but it depends on which fuel injection system the car has. If your car has the Standard K-Jetronic Fuel Injection System, then any adjustment you make for the fuel mixture, it will change accordingly. If the car has the K-Jetronic E, which is the electronic version of K-Jet, you can still make adjustments, but there are other sensors which will try to keep it in a specific area. You can still make minor changes in the fuel mixture.
Which year and model Mercedes do you have?