An openly-published protocol is one where the protocol is placed on the Internet, but there's a group that decides what will be in it.

An open-developed protocol is very similar, but anyone can simply implement a variant of something already included, and thereby expand or alter the existing state of the protocol. Every implementation that changes some existing protocol definition, is automatically a soft-fork. If at least two matching soft-forks exist, it is automatically a new branch of the protocol, because those two forks/implementations can communicate with each other and this is a communication protocol.

Communication between two members of the protocol over a new path or method = expansion of the protocol.

That is why you cannot stop someone from creating a Nostr software project that does something novel. That is why this place is crawling with devs. That's what makes Nostr special and inspiring: no centralized control.