Sodium cooled valves may have been "exotic" in the 40's or 50's but International Harvestor was using them in farm equipment in the 1960's. On the farm we had a cotton picker powered by a little water cooled IH V-8 with sodium cooled valves/valve rotators. It out lasted everything else on the machine. The picking heads were a mechanical marvel/nightmare though.
The metallurgical explanations in the essay are a little simplistic; I suspect work hardening has little to do with the failures of alumimum engine heads. It is more likely an issue of thermal stress/thermal fatigue.
I thought Porsche went to water cooling with the advent of production four valve heads. There was no longer the space left for sufficient finning to transfer the heat away plus there was more heat.
Jim