This is not correct. Dark subtraction and flatfield correction are two very different things: Dark subtraction corrects in an additive way the dark current. Flatfield is a multiplicative correction, which corrects in its strict sense sensitivity variations from pixel to pixel and across the detector. Mathematically it is the same as vignetting correction since both are multiplicative operations. But physically flatfield and vignetting have nothing in common. The first is a detector property, the second is an optical property.
My Sony F828 definitively does a dark correction, once the exposure time is above a certain limit: After the shutter is closed, it takes a second exposure of the same length, which is subtracted from the data. I would assume, that it is also subtracted by the firmware from the raw data.
It may be, that the detectors in digital cameras are quite homogeneous in their sensitivity across the detector (CCD or CMOS does not make a fundamental difference here). They presumably are bulk chips, i.e. not thinned for enhanced blue sensitivity as are CCDs for astronomical applications. Since astronomical CCDs are cooled, dark current is small and usually needs not be corrected for (except maybe for spectroscopy where the background is low). Just for completeness, what I mean, here is an example of an astronomical image before and after flatfield correction.
Above left is the raw science image, above right is the flatfield exposure, which shows a gradient across the field of view due to sensitivity variations. Below left is the flatfield corrected image, showing interference fringes originating in the thinned CCD due the night sky emission lines. Below right is the final image, where the interference fringes have been modelled and subtracted.
All the gory details are described in the EMVA standard 1288 “Standards for characterization of image sensors and cameras”.