But like, it's just openAPI with an endpoint for getting the schema, like how is that more universal than openAPI?

Most of the value lies in its differentiation to OpenAPI and the conventions it brings.

By providing an MCP endpoint you signify "we made the API self-describing enough to be usable by AI agents". Most existing OpenAPI specs out there don't clear that bar, as endpoint/parameter descriptions are underdocumented and are unusable without supplementary documentation that is external to the OpenAPI spec.