I betcha in real world non-lab conditions these things can make a difference. That is what they are designed for after all. It's not the exact same thing but watch this guy demonstrate an iFi AC iPurifier in his honest-to-goodness real life system:
As fun as it is to call everything snake oil so one can feel smart and objective- the biases about audio equipment work both ways. I think arguing that the OP's thingy would only work on cheap DACs is moving the goal post. Amir-Almighty has established there is not always a strong correlation between price and design/parts quality.
Part way through the video I was wondering if his setup was balanced or unbalanced. Then he reveals that the CMNR side of his setup doesn't exhibit this behavior.
So this leads me to me next few points/questions:
Did he ever purchase an outlet tester? That would have been my first ask.
Is the outlet he plugged the Ethernet PLA to on the same circuit?
Does the Ethernet PLA introduce any noise plugged in by itself?
If it only makes noise when the Ethernet cable is plugged in, then I would be willing to place a small wager that the cable is shield tied. And that's a no no.
This is another example of my frustration with 'audiophiles' because they are confident in their ignorance.