the plan9 port API is based on sockets, yes - because plan9port is a port of plan9 runtime to other OSes.
However, BSD Sockets are bad API, and can be argued to be one of the reasons, if not major reason, behind many issues in IP-based networks, including slow IPv6 rollout.
They also were a rushed port/rewrite of existing TCP/IP code done essentially by grad students on a grant from DoD because vendor (DEC) told DoD that they are not getting any new models of the computer they used primarily for TPC/IP.
Part of the requirements was that the resulting code is shared so that vendors could quickly provide support for TCP/IP in various products.