TV person here, I wonder if its not on the NOVA episode list because it was actually initially produced by/for/with another broadcaster? In the 80s they did license episodes of Horizon from the BBC. So could it have been something like this (from a quick google) www.imdb.com/title/tt1750...?

"Horizon" Painting by Numbers (TV Episode 1981) | Documentary

Painting by Numbers: With Paul Vaughan, Robert Abel, Jim Blinn, Nolan Bushnell.

Which I know has already been shared but it may be worth looking at both Horizon from the BBC and Equinox from C4 as I know that NOVA did license or co-produce episodes with both broadcasters - still do.

If I can get hold of the Horizon episode I’d probably recognize parts of it.