That is probably an overstatement. The mistake Hitler had done attacking the USSR was a repetition of the mistake Napoleon had done. Both believed they could pose themselves in the eyes of Russians as heroes liberating the people from their oppressors. Using the conventional military technology dated back to Napoleon or Hitler, there was no way to conquer and have control over the territory of such magnitude.
So, Stalin or no Stalin, there was no way Hitler could be successful in the long run. And, in my opinion, Stalin was greatly responsible that the war lasted that long and took such expensive toll. His impatience to defeat the enemy quickly and throw Germans back beyond the borders as fast as possible lead to a lot of losses during the entire campaign. Especially, in its initial phase.