I disagree with this. A year-long course grade carries much more weight in the admissions process than a 3 hour test. Some schools don’t even consider AP scores during admissions.
A C for the year obviously isn’t as good as an A or a B. But in the overall context of an application, a single grade isn’t a make or break item. It could be a differentiator at Harvard, MIT, etc. For UGA, a very good GPA with a single C in the mix will still put you in a good position.