"Our analysis also shows that code elements that are perceived more difficult by developers also end up having more quality concerns found in peer code reviews."
How is this surprising in any way whatsoever?
Why does it matter if it's surprising? Science isn't meant to reveal surprising things. The purpose is substantiating or refuting hypotheses. The only relevance your expectation has to a scientific conclusion is in determining sources of bias.
If you read the rest of the text they word it like it would support their hypothesis, which doesn't follow logically.
It's not but just because we think it's a no brainer doesn't mean they can just "go with it" without "proving" it.