exactly.. protocol.. meaning the rules... not the GUI.EXE not the one sided group of programmers..

any one write the same rules(protocol) and be part of the network.. it doesnt have to be bitcoin-core or nothing
as long as the main rules are followed to allow handshaking between the nodes (no matter who programmed the node).. they can all work happily beside each other as they are all using the same rules..

some people really need to realise that bitcoin-core and the blockstream dev team are not unique. anyone can replicate the code and then change things that do not affect the main rules.. and it will still work