Now that is interesting. I personally agree with the hypothesis that lisp is a step forward, but often wonder why it's an anecdotal argument than one that is data backed.
This paper although makes me think they compared programmers not languages, as the imperative language programmers didn't have any functional experience. It would be interesting to see a comparison between programmers of roughly equal competency.
Mostly it's due to a lack of good, controlled, data. The ideal study would need to be of a large and complex system that was recreated several times in several languages. You'd need to verify each system behaved the same, and that the teams on each version were nearly equally competent in their respective languages. Even that would be poor, since it would be a single data point. Perhaps a particular language was simply good at that problem!
Despite that complaint, the above is nearly impossible to do in practice. So you have to settle for smaller problems, which can be done in several months perhaps. Even at that, it would take some very difficult collaboration, and would still be a single data point.
Further complicating issues is the academic world vs. industry. Again, ideally you'd want to do the test in industry, with an appropriate team of programmers and managers. But industry doesn't want to waste time with such academic matters, nor does it want to open it's code for analysis.
In other words, it's a very difficult problem to address, despite being seemingly simple on the surface.
This paper although makes me think they compared programmers not languages, as the imperative language programmers didn't have any functional experience. It would be interesting to see a comparison between programmers of roughly equal competency.