Its confusing comparing different models. The posted picture is of the positive connection (red end). The negative is not shown which is where the IBS sensor is located near. I had the Maserati service department install when I picked up the car new.
The models are different, but the connection to the batteries is the same. What is shown on your picture is definitely negative pole. The positive pole is closed by the box with high-amp fuses and is more close to the tail of the car.
The car with IBS has only two points of connection to the negative pole of the battery. They are the negative pole of the battery itself and the connection point where you have the wire connected. All the rest negative connections in the car including any ground on the body are connected to the negative pole of the battery through the IBS. And all the additional electronics and chargers should be connected not to the battery negative pole, but to the ground connections of the car, which are done through IBS, or to the special ground connector on the IBS. It is new for Maserati, and it was introduced only in last Quattroporte and Ghibli, so the official dealers may be not aware. Big manufacturers of the car have this feature up to 10 years already and earlier I had some experience with problems caused by incorrect understanding of it on my other cars, when this feature was new for the installers.
So your installation made by official dealer is absolutely incorrect and doesn’t correspond neither to Owner’s Guide nor to technical/electrical documentation for the car.
Please find below the picture of the correct installation of negative wire for CTEK tenderers/chargers, as well as the view of the CTEK connector inside the trunk.