That is not necessarily true.
DeMar Derozan is not the greatest 3 point shooter and he is a SG in the NBA for the Toronto Raptors. He gets many of his points from driving and hitting 2 point shots. Not much of a 3 point/long range shooter. I think he is capable of hitting a wide open 3 point shot if his defender sags off of him too much. He won't make it all the time, but he is capable of making it. (In fact, I think he only averages 1 3pt shot attempt per game and shoots the 3 at about 21% in his career)
I think Marble is very similar. I think if Marble is left open for 3 that he is capable of making it. I think Marble just lacks the confidence from beyond the arc right now, but I think that will improve next year.
However, I do like Marble at PG. He really does play a lot better when takes the ball up the court and has it in his hands more. He creates good shots off the dribble for himself and I don't think he does all that great of a job creating space from movement off the ball. (At least, I haven't really noticed it.)
It will be interesting to see what happens with him next year, but I'm guessing Fran will keep him at the 2 spot and maybe put him in at PG occasionally, depending on how good Gessel and Clemmons are.