GP isn't saying that there is evidence that open offices work. GP is saying that execs want such evidence. Way back when Google was young its execs thought outside the box, so it's no surprise that they didn't copy what MSFT was doing.

Wait, so now there are thinking-outside-the-box-execs who don’t need any evidence, and regular-gimme-evidence-execs who do?

Yes, but only in young startups. Once the companies earnings go beyond a certain point they get MBAs for executives.