As can be seen VOH and VOL specifications for programmable clocks are not specified. In the Electrical Characteristics table of CY22393, for example, lists IOH with a condition of VOH=(L)VDD-0.5, (L)VDD = 3.3V. For this condition when VOH = 2.8V (3.3V-0.5V), Cypress guarantees that there will be at least 12mA of current being driven from our part. The 12mA is a minimum specification that the designers need to design to. It is a minimum value chosen by design and marketing that Cypress can guarantee the part will drive. Most likely there will not be a case where the driver will only source/sink 12mA.
The outputs are tested by setting the clock output voltage to either GND (IOL) or VCC (IOH) and using external equipment to sweep the voltage from 0 to VCC and measure the current at each voltage step. This produces I-V curves where Cypress looks to see what the minimum current is at the specified voltage. In this case 2.8V has a minimum of 12mA under worst case conditions, but normally is around 24mA.
Cypress does not specify the other way around specifying a current and state the voltage level.