Given that that judgement was made in 1981, it's possible that the judges (who were likely a bunch of depression era old dudes) had zero knowledge or exposure, and had never even thought much about, personal video recording before a bunch of lawyers tried to explain it to them during the case.

We have see this happen repeatedly with modern tech cases.

I doubt that. Home video recording, while a new thing in 1981, was not substantially different from making personal mixtapes on tape from radio or vinyl records which had been popular for decades. My grandfather had dozens of 4 track mixtape reels he made in the 60s. You could even go further back and say it wasn't any different than taking a photo of artwork for personal use. You didn't have to be that young in 1981 to understand what home video recording is.

Judges asking things that are obvious to us make for great headlines and quotes, like "what is a website?" or "what is an API?" and "shows" how out of touch they are, but like a judge (trying to) define pornography, making sure the plaintiff, the defendant, and the judge are on the same page seems to me (I am not a lawyer) just good procedure. First everyone has to agree on what a website or an API is before passing judgment on legal matters concerning them that all parties will abide by.

Even complete legal novices like me know about the Sony/Betamax case, FWIW. It would shock me if a judge ruling on copyright implications of a technology didn't know about it.