Michael Atiyah is the greatest mathematician.
He also didn't think that the Russian guy proved the conjecture. A proof should be understood by at least several top experts in the same field. Obviously the Russian guy's manuscript failed to convince others that he proved the conjecture. Of course, no one denied that the Russian guy made a breakthrough in proving the conjecture. Only after Yau announced the complete proof, then everyone in the world thought that the Russian guy proved the conjecture.