That's true only on newer phones. On Gingerbread phones, the external SD card was mounted in /mnt/sdcard and when the device was plugged to USB, the SD card was then unmounted from the device and mounted as Mass media storage to the PC.
On ICS and higher the "external storage" is in fact internal, but still mounted to /mnt/sdcard for some backwards compatibility. When the device is plugged to USB, the MTP protocol is used, so the "sd card" doesn't have to be unmounted anymore.