No way mate.
The critical word “remain binding AS between the belligerents who are parties thereto.”
The Egorka and pdf are right.
The Geneva convention of 1929 rules toward enemy pows the has been shared all over the belligerents states independently on , whatever the enemy state signed it or not.
It might make more sense if one considers the doctrine of privity of contract in civil law in English speaking (common law) countries, which holds that only the parties to a contract are bound by it. So, for example, if you and I contract that I will buy Chevan a new Lada then you can sue me if I don’t perform the contract, but Chevan can’t. (Why he would want to force me to inflict a Lada on him is a different question. :D) Whatever happens, Chevan isn’t a party to our contract and can’t claim any legally enforceable benefit under it.
But no one "doctrine of privity in English speaking countries " can’t prevent me with Egorka to catch you in ,for instance, the Neitral non-english speaking territory and treat you just like the Germans, who signed the Geneva convention in 1929, was treating the Soviet pows,
Until you would have finaly bought my “new” Lada for …only $50 000.:)
The same general idea applies to international treaties. For example, if countries A and B sign a treaty not to fish a certain area of ocean, that doesn’t impose any obligation on country C, which hasn’t signed it, not to fish there nor does it confer any right on C to stop A and B fishing there if either of them breaches the treaty between A and B.
Conversely, if the treaty requires A and B to bring any dispute between them about fishing there to arbitration, it doesn’t stop either of them sending a gunboat to eject C from the area, even if the treaty prevents them doing it to each other.
This case however did not share in situation when the A and B who signed the treaty not to fish in the neitral waters, start the warfare and blame each other in war crimes and sink each other without any limitation ( Unlimited warfare).
Doest it mean A and B can sink also and C ships just because the C has signed nothing and both A and B are obligated to folow the “rules” that C isn’t ?