It all depends of what is meant by “not a man of words”.
There are many stories about Moses’ speech defects. The rabbis say, for example, that as an infant Moses took Pharaoh’s royal crown and put it on his own head. The courtiers were aghast and a soothsayer predicted that one day Moses would destroy the Egyptian kingdom. Jethro, one of the courtiers, argued that the boy had not really known what he was doing. It was decided to test Moses.
A shining piece of gold and a burning coal were placed in front of him to see which he would choose. When the boy stretched out his hand an angel guided him to the coal, which he picked up and put into his mouth. It burned his tongue and left him with a stutter but it saved his life (Ex. R. 1:31, etc.).
Yet the stutterer became a speaker when circumstances made it necessary. The task of leading the people fired him with such passion that he became able to use words to rebuke them when necessary and to inspire them constantly.
According to Rabbi Levi (Deut. R. 1), it was the Torah that healed his stutter: “When he acquired the merit of Torah his tongue was healed and he began to speak words”.
Rashbam offers another view: when Moses said, “I am not a man of words”, it did not necessarily imply that he stuttered. It meant merely that he was not a trained orator; he was not an expert in planning speeches or in arguing a case, and he was apprehensive lest his own inadequacies jeopardised the people’s future.