I think you're too quick to discredit the claim, perhaps its not quite as strong as people make it seem but... This article written in 1981 from the NY times titled "Will Cable TV Be Invaded By Commercials" would somewhat disagree with you. I think makes the situation as clear to understand as possible:
People were assuming because they were paying for Television there would be no (or at least less) ads. The article then states that there was $45 million in cable advertising vs $11 billion in traditional. I think those numbers alone make it pretty clear advertising was not at the top of the (at the time) nascent industry's mind. And, while this is only a guess its probably safe to assume the bulk of that $45 million went to at most a handful of "innovative" (vomits in mouth) cable companies.
The first phrase in that article says "Although cable television was never conceived of as television without commercial interruption" which 100% supports my position. The original "cable TV" was to get all the broadcast channels near by in perfect quality without an antenna, thus having ads. There were ads on "cable TV" from the beginning , many consumers just assumed it wasn't there. It also acknowledges the rest of my point in stating "Indeed, even pay television, once assumed to be secure from commercial interests, is attracting some attention as a potential vehicle for advertising." Note that essentially some channels were paid extras which did not originally have ads, while other channels like CNN, USA, and RCTV did have ads from the get go.
And yeah, advertising revenues weren't nearly as huge for cable networks at the beginning. The first cable networks started operations in the 50s, and you're sharing an article written in '81 pretty much showcasing that there were only a few cable-only TV stations at the time. At the beginning there were zero cable-only TV stations, and that was how it was for about 30 years. All channels you got on cable were just the broadcast stations. Many years later they started adding cable-only stations with some of those stations having advertising and some of them without.
The first real cable-only TV channel was HBO, which was without ads. The second real cable-only TV station was WTBS, which had ads (it was a nearly nation-wide rebroadcast of Ted Turner's broadcast TV station, so only semi-only-cable?) and started on cable in 1976. After that there was Christian Broadcasting Network which was ad-free. In 1979 we got Nickeloden and ESPN (ads), in 1980 we got CNN and USA (ads).
https://www.nytimes.com/1981/07/26/arts/will-cable-tv-be-inv...
People were assuming because they were paying for Television there would be no (or at least less) ads. The article then states that there was $45 million in cable advertising vs $11 billion in traditional. I think those numbers alone make it pretty clear advertising was not at the top of the (at the time) nascent industry's mind. And, while this is only a guess its probably safe to assume the bulk of that $45 million went to at most a handful of "innovative" (vomits in mouth) cable companies.