Why are slotted holes shown as round holes rather than slots

The pins are defined as round in the viewer and in Altium because of a limitation in the file format which doesn't allow us to define non-round holes. However, we export a mechanical layer that includes the slotted hole dimensions.
When all the layers are sent in the Gerbers to the PCB manufacturers, the slot overrides the round hole, and it produces a slotted hole in the PCB. We have a tutorial on how to properly map the layers upon import here: http://blog.snapeda.com/2018/06/22/how-to-map-the-footprint-cutouts-to-the-proper-layers-in-altium/