I'm not sure what you mean by that. if you want to interact with users hosted on bluesky you do need to connect to bluesky servers. the protocol and software is freely licensed, and folks can use it how they want. there could be more service providers, but that is different from being "open"

@bnewbold.net well, if you look at the ActivityPub protocol, you don't need to use a specific companies servers to interact with AP users....that is open, I have a few dozen *software* options to choose from when it comes to ActivityPub. ATProto has BlueSky.