Impossible to settle? Probably. Impossible to discuss? No.
I'm not being critical of your account, I'm glad it has worked for you, as the manager of the engineering effort here I want to take the "good" stuff and use it, and leave behind the "not so good" stuff. So that is my agenda in this discussion, nothing more, nothing less.
"In general the people I've seen take 20% time in a focused way tend to be the higher-performers."
That is my experience as well, and regardless of a company policy for or against 20% time, focussed, high performing engineers will spend 20% of their time do cool and innovative things. Whether they do it at home or at work doesn't really matter, and having them do it at work is good for the company.
So the interesting thing about the policy is to capture the next tier of engineers and help them to be more productive by encouraging them to develop habits of the aforementioned highly successful focussed engineers. And to weed out the folks who are abusing the program [1] or at least not being any more productive with it.
If nothing else people are different right? Facebook's response has been "hackathons" which carries with it some characteristics of high performers (who quickly prototype ideas to test their validity or get a handle on their challenges)
But in all those scenarios, if you have managers, you need to also train your management on what the program is trying to achieve and how it might be addressed. So you don't end up with some managers giving their people 1 day a week off, and some demanding they work on Saturday if they want to use that extra time.
If it is "You have this huge resource available, dare to use it." then you can manage to that without damaging either morale or perf scores. From the anecdotes in the OP article it sounds like they are still working on that part.
[1] Like the guy who said he was trying to capture the great ideas he dreamed about in his 20% time so he would spend several hours napping for an hour and then waking up and writing down what he dreamed about.