Nobody would have to believe him but he could have decided not to go to war with the US. History is replete with politicians, especially despots, telling big lies. At the very least he could have privately demanded that Japan attack the Soviet Union as the quid pro quo for Germany going to war with the US. Remember that there is good evidence that Stalin put out peace feelers thought Japan and Sweden in the spring of 1943. If the US were not participating in the war against Germany and was supplying less equipment to the Soviets' and British peace talks in 1943 may have progressed further.No-one in their right mind would believe Hitler on that. He has invaded several countries with sneakattacks, so no, it wouldn't make a big difference.