That wall of screen was a tradeshow display -- by Google, of course. But check this link (it's in the intro text) again: https://groups.google.com/foru...
The idea isn't that every Web designer in the world should have his or her own wall of screens, but that you and other people who make sites and games and such might collaborate on setting up a group of displays that includes some of the most popular OSes, browsers, and device form factors.
I have always been shocked at how many people who make websites design for a browser, OS, and screen size just like theirs. I remember a conversation in 1998 or so with with a web designer who said, "But our target audience is like you and me - they all have big monitors."
I said, "Really?" and hauled out my little laptop. "What if I'm looking at your site in a hotel room someplace instead of in my home office?"
"Oh," he said.