Here is how RFC 7230 defines "HTTP/1.1 pipelining":
"6.3.2. Pipelining
A client that supports persistent connections MAY "pipeline" its requests (i.e., send multiple requests without waiting for each response)."
AFAIK, the curl binary does not do pipelining by this definition.
And, AFAIK, it will not generate code to do pipelining by invoking it with --libcurl.