It would be possible to connect a skimmer to the main drain, but I can't imagine someone doing so. More likely the open side port is (or was) connected to a skimmer bypass, that prevented the pump from running dry if the water got too low to fill the skimmer throat. It's become common to plaster over the sidewall opening to which that port would have connected.

By the way, you REALLY should get a skimmer basket. If the wrong plastic toy or object ends up in the skimmer, you'll end up digging up the deck to remove it!