Interesting experiment… but I would be a little careful with the conclusion. You are not actually running the system directly from a battery. You are running it from a battery feeding a switching inverter which regenerates AC. “Pure sine wave” is a good start, but it does not tell us much about the inverter’s RF noise, common mode behavior, output impedance, or grounding scheme.
At roughly 80 watts, the EcoFlow has abundant capacity for a streamer, DAC, and preamp, so there is no reason it could not work well. Still, others who have tried UPS and inverter power have sometimes heard the opposite… reduced dynamics, a little hardening, or generally less natural sound. It seems very dependent on the particular inverter and the particular system.
The change you hear may be quite real… but it may be less a matter of “battery power is inherently cleaner” than the charger being disconnected, or the grounding and noise paths in your system changing. The front end is not necessarily fully isolated from the house because the interconnects to the power amp, and possibly Ethernet to the router, can still provide paths back to ground.
I would also not say voltage regulation and protection are irrelevant. The grid is out of the picture while unplugged, certainly… but the inverter is now the voltage regulator, and its behavior becomes part of the system. Still, if repeated comparisons continue to show the same reduction in sibilance, then you have found a useful solution to a real problem in your particular setup.

