Why Stalin's participation could collapse the treaty

Would changing the participants produce a treaty too different to serve as an equitable version of the historical settlement?

Yes. In the Stalin example, Soviet demands for parity with Japan or the Anglo-American fleets could prevent agreement rather than produce an equitable version of the same settlement.

Changing the participating countries changes the assumptions of the negotiation. Stalin's attendance supplies the counterfactual example: he would have wanted at least parity with Japan and might have demanded equality with Britain and the United States. Those demands are expected to disrupt the compromises among the historical participants. In this hypothetical, adding the Soviet Union could prevent agreement altogether.