Does DisplayPort support scaling natively?
A DisplayPort cable simply transmits electrical signals without changing them.

Objective 3) just says that a display that is not capable of scaling is DisplayPort compatible. In other words, the DisplayPort specification does not require that all displays must support scaling.

If you have a display that does not support scaling, and if your source is not able to generate images in the correct resolution, then the system will not work. Allowing this failure mode was seen as an acceptable tradeoff to avoid higher costs.