I don't see why not; serial comms (over whatever medium) connecting two
machines is still 'networking' in my book. I the case you mention it may well
be only point-to-point rather than "lots of machines on the same line", but I
still think it qualifies.
Then let's go a step further. When/where did the first internetworking
gear come onto the market? Not multi-protocol routing, just routing in
general: segmented networks with nodes that moved traffic towards remote
nodes on the behalf of local ones.