I don't know, I think that one's justifiable. In the pilot the crew had just found out he wasn't who he said he was. Simon doesn't know much about them, doesn't know how much he can trust them. So he tells them a story which is close to the truth without giving them the impression he might be in any way dangerous.
Not that he *is* particularly dangerous, mind. But he probably thought of himself as dangerous after that rescue stunt!
I'd buy that if his characterization in the rest of the series didn't match his story so well. Simon is never the man of action the beginning (as well as the rest) of Serenity