I'm going to assume the actual statement "Good students are good at everything" is a quote that is, at best, rather out of context. It's patent garbage; I often take classes (and advise others to take classes) in things I know I'm no good at, either to advance from "Useless" to "OK", or just because it's something I enjoy. My art skills are truly awful and I'll never get top marks, but it wouldn't stop my taking an elective, because I'd like to be better and enjoy the area.
People who constantly get top marks in everything are, to me, suspicious. Mostly anyone who's smart can pick a syllabus they'll ace - but will they gain anything from it? Now, if they're getting constantly low marks, or random mark disparities in seemingly related areas, that might be a legitimate concern, but to blanket it as "Good students are good at everything" is just bizarre.