It's a stylistic opinion rather than a hard distinction. Yes, in both cases you're exchanging messages and responses between turing-equivalent systems. However, the systems you end up with are different depending on which pieces are designed first and with higher priority.
NFS is an RPC system. Both protocol-based and RPC-based systems can be well-documented and well-specified. Both kinds of system can be arbitrary nightmares. I don't think you're drawing a useful distinction. Both ecosystems have some services provided over RPC and some services provided over some kind of other message format.
> [RPC] requires a server architecture identical to the MS one
No it doesn't. It "requires" Exchange-style architecture in the same way that IMAP "requires" maildir storage. In the end, you have a message-passing protocol, and you can choose to respond to messages any way you'd like.
> GNOME have been able to produce such a bad wire protocol for dbus because they're writing both ends and happy to break compatibility regularly
They claim that the dbus wire protocol is stable and that individual dbus services will present broadly compatible interfaces. Dbus is for everyone, not just internal communication inside GNOME. That's what makes dbus being awful especially annoying.