The individuals may have been more productive; did they measure the output of the teams?
I don't think anyone disputes that people are more productive without distraction. That's not what open plans are for though: collaboration helps everyone orient around the right thing, so all that individual productivity isn't wasted pursuing the wrong stuff.
Their discovery was that people who had closed door offices were more productive. In general people with less interruptions got more done.
The book is somewhat dated, so this may have changed since, but one of the interesting things they found was that there weren't really any studies showing that open offices helped productivity for software teams. It was purely a cost saving measure by execs wrapped up in wrapping of "being collaborative".