The quality of a detailers work is not defined by the fact that they arrived at your location any more than it is if you were to drop your vehicle of at their location for the work to be carried out. Some people will say that a fixed location is cleaner, this may or may not be true. But if your garage is clean or I clean it and have the door closed then we are both working in the same kind of environment. The end quality of the paint finish is more to do with the products and equipment used and the skill of the detailer who is carrying out the paint correction than the physical location of where the vehicle was detailed.
BMW paint is pretty bad for this, the paint texture resembles that of an orange and is caused by a number of factors typically the result of improper painting technique, quick evaporation of thinner, incorrect spray gun setup (e.g., low air pressure or incorrect nozzle), spraying the paint at an angle other than perpendicular, or applying excessive paint.
The only way to remove this is by wet sanding the paint, some people will tell you it can be removed by polishing with a rotary, this is incorrect as the soft foam pad will conform to the peaks and troughs of the orange peel. Wet sanding is a long expensive process so it may be easier to just live with it.