Vochindik, that can mean two things. It can mean that gedolim never did anything that could possibly be criticized, which would indicate that that you’ve read too many Artscroll biographies yourself.
Or it could mean that since their gadlus cannot be called into question, analyzing the specifics of what they did, whether it seems to be something one would agree with or not, can never be negative. That is a more plausible view.