This is true. Mafia is often a game of persuasion. I can't begin to tell you how many times I've had to persuade a town really hard to lynch someone who is obviously scum (go see the archives!) but it was a constant source of frustration. Either way, from that experience I can tell you firsthand people are likely to believe someone who is confident about their target being scum than a wishy washy "oh they might be, I could be wrong though" half-answer.