Could very well be. I'm not really sure if other problems would occur on these being overfilled. IE will it whip itself into a frenzy like an engine would whip air into the oil and not lube or drive properly . My guess that it isn't too likely. Guess... Can you draw some oil out of the vent or are there signs of if pushing out oil there?

If the gear box is not vented and the oil level covers the axle bearings, it would be likely to leak as it warms up with use and the oil expands. Maybe there are seals that can be replaced? I think I would use it and monitor the situation. The leaks will stop after the oil level gets below the axle bearings. Then the bearings will run dry except for splash lube but that may be what was intended.