Not the first time this has been suggested; what it turns on, practically speaking, is the state of Russian roads at the time. The spring thaws- the rasputitsa- convert most of them to rivers of mud, and it is only around midsummer they become really viable again.
That is the basis for the counter- claim that the German army would not have moved any sooner than it actually did, even without the Mediterranean to worry about.
It would actually have been counterproductive to do so- the ground was in no state to support a war of movement, and there would have been significant advantage to the defender, blitzkrieg would be harder and less practical, the lids wouldn't have closed on the cauldrons (kesselschlacht, not cookery), and, in general, worth waiting.
Hitler's prediction belongs alongside "we have only to kick in the door, and the whole rotten edifice will come crashing down"; you can just about see why he might think so, but also that there is daylight showing between his ideas and reality.