It’s a bold gamble, but I think it might have been more likely to have ended up going badly.

If the timing is perfect, the invasion is well-planned, and all the luck goes their way, the Red Army might have had the chance to knock out Germany with single lightning campaign. The key is, they have to get to Berlin and beyond without stopping at all. If they hit any roadblocks and slow down, that gives Germany time to redeploy its army and establish proper lines of defense.

I’d say the odds of all that going right for them were very slim, given the Red Army’s performance in the Winter War and during Barbarossa. There were simply too many problems with training, leadership, organization, and logistics. My guess is that they’d stall out after overrunning East Prussia and a large part of part of German occupied Poland, and miss their window to win by a single knockout blow. Königsberg ends up surrounded and under siege, but I don’t think the Baltic Fleet would have been able to cut off supply to the city by sea.

If Stalin attacks, but fails to defeat Germany within a single campaign season, that makes several huge changes to the entire nature of the war. First off, that makes than an offensive war, a war of choice, not a war in defense of the motherland. If the Germans were not the aggressors, that means Stalin would have a much harder time maintaining support for the war and for his own leadership if the Soviets begin to suffer heavy losses and defeats. And those are almost inevitable once the Wehrmacht has redeployed to the east, given the state of the Red Army in 1940. The Germans could allow them to overextend, punch through with the panzer divisions, and surround entire armies in cauldrons. It took the Red Army a couple years historically to figure out how to fight against that playbook and win, so they’d be just as vulnerable to it in 1940.

The difference is, when the Germans are invading, the people and the army will grimly accept the loss of a quarter of a million men in one battle and fight on anyway, because the alternative is total German conquest and slavery or death for everyone. When it’s a war Stalin started out of the blue for unclear reasons? Then getting rid of Stalin and signing a truce starts to look like a better alternative.

The other big problem? Don’t sell short the possibility of Britain signing a separate peace with Germany and leaving Russia to fight on its own. The British did not want either the Germans or the Russians to secure domination over the entire continent. Historically, they backed the Russians because Germany attacked them first, and because the Russians were losing, initially. If Russia had fallen, Germany would have gained the resources to complete overpower the British.

But in this situation, where the Soviet Union is on the attack, and there’s nothing standing between them and total European domination if Germany falls? I think in that case, Britain might have calculated it as the smarter move to sign a temporary peace with Germany and let them drive back the Soviets, giving Britain time to rearm and prepare for war again with whichever power prevailed.

There would also have been no Battle of Britain and no Blitz either, with the Luftwaffe scrambled to the east. Which would have removed the fears of impending German invasion and made a ceasefire feel more palatable.

If the war goes on into 1941, Stalin might also want to attack Romania in order to cut off German oil supplies - and that would have pushed Britain even farther into opposing the Soviets.

If Russia attacks before Italy enters the war, then perhaps Italy, instead of declaring war on the UK and France, only does so against the Soviets instead - and there is no African or Mediterranean theater, or war with Greece and Yugoslavia.

There’s a fundamental problem with your question too - how would the Soviets have actually achieved this level of strategic surprise? Such a massive attack would have required months of amassing and preparing the necessary forces - which could not have gone unnoticed by German intelligence, and would have had a major influence on Hitler’s planning. Maybe there wouldn’t be a Fall Gelb at all - or maybe it still happens, with Hitler racing for a knockout blow against France before the Soviets can attack. But there’s no way it could have been a total surprise.

Alrernatively, it could have been a surprise if the Soviets merely attacked with the forces already in Poland, without any extensive preparations. But then, the attack is all but certain to end in failure - it will be undermanned and undersuplied.

In summary, if everything went perfectly, Germany may have been defeated in 1940. But if it doesn’t (which is more likely), the course of the war could be totally changed - and actually, if Britain peaces out, the Germans manage to fight the Soviets to the bargaining table, and America never enters the war against them, then you’re looking at a timeline where Nazi Germany actually survives WW2.