PSoC doesn't have a voltage DAC, VDAC8 is emulated using current DAC, attached to a resistor (16k @ 4.08V range). Therefore output voltage of such DAC is V = I x R in the absence of any load. If load impedance is not infinity, then output of VDAC8 is V= I x (R || Rload). For that reason datasheet requires load impedance 'purely capacitive', that is infinite resistance.
The rule of thumb is that any VDAC8 output going outside the chip must be buffered. When using PSoC built-in Opamps for buffering of the VDAC8 output, please note that the input impedance of the Opamp is not infinite (it is about 1M), which slightly affects the output voltage when VDAC8 range is 4.08V (R=16k).