I do not see any double counting, just simple arithmetic. The links are full duplex, thus 600 GB/sec of bidirectional bandwidth per link. 4 links at 600 GB/sec result in aggregate bandwidth of 2.4 TB/sec. No trickery there. Now: Is that number meaningful in a practically applicable way? I suspect not. But I am not very knowledgeable about such big systems, they are not something I could afford.
It is the middle of the night in the US right now, so my suggestion would be to wait for an authoritative answer from an NVIDIA employee.