I would guess "it doesn't impact the quality of responses" was guaranteed to be claimed before they even implemented any of the watermarking.
And would come from marketing, not the people who implemented it.
If the model’s most recent output is “for (let i = 0; ”, the likelihood of the next token being “i” is probably millions of times greater than any other possible token. Thus even if “i” is on the red list and has its likelihood decreased, it’s not going to suddenly choose another word.
Put another way, on low-entropy tasks like coding, this style of fingerprinting is less effective and needs bigger sample sizes to be recognizable.
That said, even small changes can dramatically affect output quality, which is why I’m still a skeptic.