Yes, the decoder board itself has no solder pads. It only has a small connector to which the adapter board (with pads) is connected.
I'm wondering if the initial 58923 came with a smaller adapter board (the one shown in all the ESU literature)? If that is the case, the ground pad is not shown on it. But on the larger adapter board the ground pads are clearly labeled GND (as shown in Steve's post).