when you command + click a buffer don't you see this contextual menu? it shows all open files, grouped per mode

I can switch to the buffer menu but there are only the opened buffers visible. I have the assumption that LSP is opening files in the background which is not visible there. For example when I open a PHP file there is a warning about watching the whole folder will reduce the speed of emacs.

maybe this rings a bell for #lsp folks

I'd be inclined to say that if a buffer has been opened by Emacs then it's visible in that contextual menu if it's LSP that it's doing the opening, that's an external process, so Emacs won't be exactly aware of those files