How are they both describing the remote procedures and parameters tho? In order for the LLM to use a tool it needs to know its name and arguments. There has to be some kind of spec, in some or format, for it to use.
An existing Swagger/OpenAPI spec is not sufficient. You want to limit options and make it easy for an LLM to call your tool to accomplish goals. The complete API surface of your application might not be appropriate. It might be too low level or require too many orchestration steps to do anything useful.
A lot of existing API's require making additional calls using the results of previous calls. GET /users to get a list of ids. Then repeatedly call GET users/$id to get the data. In a MCP world you would provide a get-users tool that would do all this behind the scenes and also impose any privacy/security/auth restrictions before handing this over to an LLM.
We see similar existing systems like GraphQL which provides a fully hydrated resultset in one call. Tons of API's like Stripe (IIRC) that provide a &hydrate= parameter to specific which relations to include full details in-line.
I do agree MCP is overhyped and might not be using best principles but I do see why its going off in its own land. It might be better suited over different protocols or transports or encodings or file formats but it seems to at least work so until something better comes along we are probably stuck with it.