(Speaking for myself, not for PS Audio)
Most UPSs don’t make anything near a sine wave, the cheapest make square waves, the slightly better make few steps to approximate a sine and most of the ones that claim something like “True Sine”, etc. are just a better approximation, but still with many small but sharp edges, those edges are a lot of noise, if you filter the edges out you loose the ability to respond quickly to changes in current draw. Most audiophile power supplies take huge gulps near the peaks of the input power so they are very sensitive to the instantaneous current available.
(See https://forum.psaudio.com/t/power-cord-for-dsd-sr/5445/21 or https://forum.psaudio.com/t/power-cord-for-dsd-sr/5445/30 )
There are some audiophile online USBs, I have one and it isn’t bad for low current devices and I never hear it’s fan, but for amps the efficiency of double conversion is too low and the lack of instantaneous current delivery would be a real problem.
Note that the Tripp Lite 1000VA Smart Online UPS has different outputs: “Pure Sine Wave” in “Battery Mode” and “Sine Wave” in “AC Mode” And it’s output regulation is +/-2% in “Line Mode” or “Battery mode” and +/-10% (!) in “Economy Line Mode”. What they mean by Pure Sine Wave and Sine Wave isn’t clear, but 2% distortion leaves a lot of room for high frequency edges.