They didn't reinvent it, it's literally using JSONRPC. The thing they "invented" was coming up with a loose standard for how to implement your RPC endpoints in a way that the AI was pre-trained to interact with.

The actual implementation may have been very janky but the idea itself is sound: If you want to interact with arbitrary services then providing one single protocol (ideally not JSONRPC but hey) with a standard api discovery method and standard starting method call means your super-intelligent machine god can connect to any MCP service with just a URL and figure everything out from there.