This is is an alternative theory that has gained traction in some contrarian quarters; but holds little weight.

The Soviets invaded territory that the Japanese held; and their actions did remove a possible pathway for Japan to stay in the war, sure.

However -

The US was the main game in the Pacific Theatre and their continuous, rolling campaign of victories - culminating in the bombs - was the primary cause of the end of the war.

This is the general consensus and has been revisited.. and revisited .. and revisited .. mostly by those with a primary motive of discrediting the US for using the bomb.

It’s easier to argue against the use of the bomb when you bluntly reject any positive outcomes; but first one must deal with reality. And the reality is not difficult to discern.

The Soviets invaded Manchuria, meanwhile the US had already taken actual Japanese territory.

While this absolutely made it much more difficult for the Japanese to allocate the resources needed to defend themselves on so many fronts, they were far more interested in defending their homeland than in keeping captured Chinese territory.

If you were to consider two counterfactuals -

- Soviets invade, bombs are not dropped - Soviets don’t invade, bombs are dropped

Only one of these still leads to surrender. Manchuria was a sideshow compared to the real game.

Editing to add -

If you want any further proof, just look at who the Japanese actually surrendered to. There is nothing in their culture (then or now) which would cause them to surrender to anyone besides their actual victor.