Not necessarily, but he was very clever, and no-one thought in quite the way that he did at the time. His insights into theoretical physics were ten quite unique. The reason why he was considered such a towering intellect was that he was the first to think them, and no-one else could quite get their head around his ideas. Max Planck, Nils Bohr, Ernest Rutherford and Isaac Newton have each also been considered the smartest person in the world - notice that they are all physicists.
