> LSP is an API that talks to compilers, and returns actual relevant data about the code you are working with
That's what YCM did as well. Well, compilers or other intellisense-adjacent tools (like Jedi for Python, which Python's language server also used, or Tern for JS).
YCM includes various non-standardized static analyzers and such, and glues them together to be a swiss army knife. LSP is a standardized API that compilers can target, and that means there is no need for a middleware component that does work.
That's what YCM did as well. Well, compilers or other intellisense-adjacent tools (like Jedi for Python, which Python's language server also used, or Tern for JS).