I'm a fan of that phrasing usually but that's not a good comparison in this case I feel. Agents are perfectly fine to use existing REST APIs. I have yet to see the big selling point for MCP as they are usually a very thin layer on top of an existing API just with a formalized description for each tool.

How is it better than just a REST API with a openapi spec json file?

you can also put that REST API behind a self-documenting CLI. that’s how I setup my agents to talk to each other