The "unknown asshole" is an IEEE life fellow and former board member, with a long successful career in research, industry, and teaching in computer engineering, and authored a widely used award winning textbook on digital design.
His profile is pretty typical for expert witnesses. Quite a large fraction of them are people who are retired (Rhyne retired in 1998) after having had a very successful and distinguished career in engineering or science, usually including research, industry, and academia.
Expert witness gigs are a nice way to pay for a new boat or RV without a lot of work. (One of the experts at the trial I was involved in was paying for a Tesla from a couple expert witness jobs). Many become registered patent agents, and then they can testify as both experts on the engineering aspects of the patent and on the patent aspects (e.g., prior art, infringement, interpretation of the specification, and such).
The workload is actually pretty light, compared to what their load was likely like before they retired. Basically, they write a report that analyzes the patent and the alleged infringement and explains why the patent is valid and why it is indeed infringed, get deposed, and testify about the report.