Where does it state that Captain America is "as smart as a human can be?" I think that it is just assumed by many since most of his other attributes are peak human. In his character profile on Marvel's own official website it says "had a very high intelligence," and on ComicVine it says "is a quick learner." We know he's not as intelligent as Tony Stark, Hank Pym, Bruce Banner, Hank McCoy, Reed Richards, Victor Von Doom, or Charles Xavier. You can leave Professor X and Beast off of the list if you want since they're mutants, but the rest were born human, and those are only some of the MARVEL characters that are smarter than Cap for a fair comparison. Even on this site Bruce's intelligence is ranked higher than Steve's.