Most of those sources have little to no academic credibility and can be dismissed outright. Also, we don’t really need the excuse of Manchuria to get IS 3 because the rules the devs set for themselves mean it’s completely fair game to add to add it to Berlin (the devs have stated that their criteria for adding things is if it existed in the time period of the campaign, they can add it).
Honestly the Invasion of Manchuria would be cool, but I have a feeling it would be difficult to balance late-war Soviet kits with anything the Japanese have