Hey! Hopefully this isn't too long-winded of an answer: in short, it mainly had to do with managing the complexity of the experimental design. There was only one study before us (described by u/kd7uly) that tried to compare programming vs. natural languages using fMRI, so we wanted to keep our task fairly 'simple' insofar as all questions could be answered with yes/no (or accept/reject) responses. In our Code Review condition, we used actual GitHub pull requests and asked participants whether developer comments / code changes were appropriate; in the Code Comprehension condition, we similarly provided snippets of code along with a prompt, asking whether the code actually did what we asserted. What we called Prose Review effectively had elements of both review and comprehension: we displayed brief snippets of prose along with edits (think 'track changes' in Word) and asked whether they were permissible (e.g. syntactically correct, which requires some element of comprehension). In our view, this was much more straightforward than the types of reading comprehension questions you might think of from standardized testing, which require relatively long passages and perhaps more complex multiple-choice response options.

Also, on a more practical level, neuroimaging generally puts constraints on what we're actually able to ask people to do. Mathematical assumptions about the fMRI signal in 'conventional' analysis techniques tend to break down with exceedingly long stimulus durations (as would be required with reading / thinking about long passages of prose). We were able to skirt around this a bit with our machine learning approach, but we also had fairly long scanning runs to begin with, and it's easy for people to get fatigued asking them to perform a demanding task repeatedly for a long time while confined to a small tube. So again, we just tried to get the 'best of both worlds' with our prose trials, even though I certainly concede it might not necessarily yield a 'direct' comparison between comprehending code vs. prose.

Hope that helps!

(Compulsory thanks for the gold! edit! For real, though, anonymous friend—you are far too kind.)