I've actually been meaning to try out fiberglass rods or tubes, but haven't felt like I needed to worry much about the weight yet. The design (as in the notes) uses a steel threaded rod which definitely holds the strength. The PLA plastic is definitely strong on its own and flexes much less than ABS would. Probably used more plastic than needed with the steel rod in mind. I think if I made a bunch more I could figure out the right balance between plastic and either types of rods.
Maybe we can crowd source the right answer over time if people post their combinations and results to share.