But much better than SMPS (or linear PSU) as they are 100% de-coupled from mains power issues (ground / leakage current loops and associated mains RF).
This can sometimes (depending on system) be much bigger issue than voltage regulation (which gets sorted inside the DAC eventually).
I know SMPS issues don't show on S2D measurements shown so far, but the sound seems smoother running on USB powerbank than SMPS to my ears... Re-connecting SMPS makes things sound a little brighter (again, subjectively).
I've found this with many 5Vdc powered DACs, not just S2D.
Even Chord Qutest - Rob Watts like powerbanks for the same reasons:
Ideally yes, ultra low noise linear regulated battery power would be best but you may already get some immediate benefit by de-coupling from AC power, even with USB powerbank...