Agreed, at least from my brief description, which was a summary of a summary out of a book. I'd say it's more likely that it's my own retelling that's flawed, though I haven't read the actual study. They also didn't have a group which was given the same number of calories from a different source (as far as I know) so it's possible that eating a chunk of meat would have the same effect. What I thought was interesting actually was that they were able to demonstrate beneficial effects from intake of calories directly on brain function.

As I wrote that, I did remember a bit of evidence that the glucose was actually beneficial - namely that they measured performance on the problems over time. At the end of the first set, both groups were showing approximately equal degraded performance. Upon resuming on the second set, the group given glucose was back up to nearly the starting level of proficiency, while the group given no glucose was at the same level they stopped at on the first group.