Schema validation performed by the REST Proxy

What is the extent of schema validation performed by the REST Proxy? This Stack Overflow post implies that the proxy only validates schemas when they are embedded directly into a request.

Whereas if you merely reference a schema by using e.g. value_schema_id, the validation is deferred to the brokers instead.

Is this a correct understanding of the behavior of the REST Proxy? I cannot find this being stated anywhere in the documentation.