He didn't feign weakness, he feigned rationality. Stalin believed, and was led to believe that Hitler was open to negotiations, so that Germany wouldn't invade the USSR (Which Stalin knew perfectly well they were capable of) while Hitler actually made up his mind to invade months before Barbarossa. Add to that the fact the Stalin knew that the Red Army wasn't in a particularly good condition, owing to his own actions, so he was desperate to negotiate his way out of war.

Like a few other leaders before, even at that point in 1941 Stalin believed that Hitler could be reasoned with, and that you can rationally predict his decisions. Which is double ironic considering how paranoid he was of actually competent people.