I agree, IMO the same effect is here - 120R of the mux vs. the 10k of the other resistor, times the mux THD => 0.2% THD * 0.01 ratio => some 0.002%, rough calculation. Absolutely inaudible. Yet IMO worse than performance of a good DAC chip.
IIRC Ivan talked about using FETs as input attenuator switches in his initial versions of his Cosmos Scaler attenuator, unmeasurable distortion. In the end he used relays anyway
https://www.audiosciencereview.com/forum/index.php?attachments/dsc_1027-1-jpg.304240/
Back to that attenuator - probably it's not about distortion, but about whether this type of analog attenuator can further attenuate noise floor from a good DAC. If not, IMO no technical reason to avoid digital VC and complicate the design with these analog circuits.