A few reasons have been previously discussed: plenty of diff viewers highlight non-standard characters being a big one, the other bit being if someone does see this error it seems entirely plausible as a
mistake.
Choosing something atypical in a way that is clearly trying to be invisible rather than being an accident will draw a lot more attention than “oops I must have typo’d it”
It was checking if the C compiled to detect the landlock feature, and there was a single character in the middle of the code that made it always fail to compile and therefore silently leave the feature disabled.