Why a Japanese submarine is unlikely

Could a Japanese submarine have been involved in Sydney's loss?

Unlikely, given the location and range involved.

Some conspiracy accounts insert a Japanese submarine into the Sydney-Kormoran action. That involvement is judged unlikely given the location and range involved. Kormoran's crew are still treated as sincere witnesses, despite questions about Sydney's later disappearance and loss of all hands. Rejecting a submarine explanation leaves the final sinking unresolved.