A curtain drain is known as a few things....french drain, curtain drain, and weeping tiles.
They all serve the same purpose, to direct water to an area where it can be controlled or pumped out. The hole you have is for the same thing as your sump pump is doing. that is why I am wondering why they did not inform you of the issue.
The only other thing it could be is a drain to the exterior sewer system or other drainage the town built in, Why did they install a sump pump when this hole?drain may have been available?
This is why I said get the real boss out there.