Not quite answering your question, but the man page says
Conflicting lock combinations (i.e., a read lock and a write lock or two write locks) where one lock is an open file description lock and the other is a traditional record lock conflict even when they are acquired by the same process on the same file descriptor.I think that answers it. And looking back at the specification in the SUSv5 draft, it seems this and analogous behavior has been codified--an OFD lock conflicts with a process-private lock and vice-versa. So there's only one reasonable option for mapping OFD locks to NFS locks, which is that they both map to the same NFS server lock in the same manner (unlike BSD flock, both support record locking).
OFD-owned locks a cross of BSD flock and POSIX locks. An original proposal is described at https://lwn.net/Articles/586904/, but the naming convention settled on OFD-owned (open file descriptor-owned) rather than file-private. I believe Linux was the first to get an implementation. They also included in the latest POSIX/SUSv5 draft; "POSIX locks" will become ambiguous in the next year or two.