It clearly states center of mass of the selected face not of the body which the face is connected to.
I agree that the help file page was written by someone who has no understanding of the concept of “face”.
That being said there is magic button on the same page, which you could use to make them aware that a face, having no thickness, thus no volume, thus no mass, cannot have a center of mass.
Also tell them what a centroid is.
From SolidWorks help. Or do you need better clarification?