The ADC in the PIXI uses a sample and hold circuit with a capacitor like what is used in a conventional ADC. The reason that the input capacitance is not reported in the datasheet, like you see for most ADCs, is because the signal will first be inputted to the PIXI Port Manager instead of directly into the ADC like you would normally expect.
We do report is the ADC acquisition time which is the time required by the capacitor to change its state from sampling to holding. This should be like what you would expect to see for a conventional ADC.