OK for idiots like me, here's the subtile part:
Since 7.21.7, this option is superfluous since you can specify a socks5 hostname proxy with -x, --proxy using a socks5h:// protocol prefix.
h following the
socks5 tells curl to lookup hostnames via proxy too.
To make curl looking up hostnames via proxy one has to export:
compared to a proxy that will lookup locally: