You're right. The output number for productivity calculations is based on GDP, so it excludes production overseass by US owned companies. The input is based on current employment statistics.
I'm not sure what Roach was thinking when he made this comment. It's hard to say how productivity would be changed by keeping manufacturing work over here, since the measure of total factor productivity includes weighted measures of inputs in a given sector. But then again, the point of moving work overseas is not to increase domestic productivity, but to increase profit.
Christian