No it's not true. It's all about how you think and understand, or, on a lesser level, how you revise, for the exams. Simply blindly putting in effort may not be enough to get more than C for some people. But the difference between those who can slave away for months to get C grades and those who get 98 UMS with revision limited to past papers is that the latter have a better grasp over how to think about and understand their subject and/or exams.