One can debate whether the Soviets would have been able to invade the Home Islands, or whether this would have been necessary to press the Cabinet into surrendering.

But it does seem strange to say they were not in a position to make a "meaningful" entry by that point, given the order of battle employed in the Manchuria campaign (per WP):

    1,577,725 troops
    27,086 artillery pieces
    1,152 rocket launchers
    5,556 tanks and self-propelled guns
    3,721 aircraft
    Mongolia:
    16,000 troops