If you read the paper, they explicitly define "productivity" as real sales per employee. Your point about service versus food would be valid except that, apparently, they weren't being paid well for the service, because real sales went up per employee as a function of reduced dwell time. That means that service was a low productivity offering.
That you can explain the increase in productivity by shift in mix from low-productivity to high-productivity products is, in fact, the entire point of the paper, and does not mean that a "surge in productivity" did not happen.
The authors point out that a change in consumer demand preferences is in fact a type of "technology" in that it can be a driver of sector productivity. Not all technology lives on circuit boards!
In the US, service is paid almost entirely from tips. The word “tip” doesn’t occur anywhere in their paper so I’m not optimistic that they accounted for this.
That you can explain the increase in productivity by shift in mix from low-productivity to high-productivity products is, in fact, the entire point of the paper, and does not mean that a "surge in productivity" did not happen.
The authors point out that a change in consumer demand preferences is in fact a type of "technology" in that it can be a driver of sector productivity. Not all technology lives on circuit boards!