I think even by ’90s standards these Saturns are pretty bad! And ultrasonic content can still cause audible problems in analog singal chains (intermodulation).
The datasheet for the ADC used in the OSSC basically says that you shouldn’t need an AA filter unless there is a lot of out-of-band noise. It doesn’t specify what counts as a lot!
My VA7 Saturn uses as a DAC a Burr-Brown PCM1710U. Looking at the VA13 schematics they’d switched to a PCM1717E. Both are designated IC22. Both seem to have the same skimpy low-pass filter. Perhaps the (newer) PCM1717E has better integrated filtering?