Presumably when prewett said "When the Japanese imported it, they used the characters much more phonetically," they meant onyomi. (Perhaps they didn't know about kunyomi.) When they say, "but the characters got assigned to the Japanese pronunciation of the word," that is mostly wrong as a description of how Japanese writing uses kanji today, but I think that is the process from which kana came: https://en.m.wikipedia.org/wiki/Man%27y%C5%8Dgana and apparently it isn't quite extinct: https://en.m.wikipedia.org/wiki/Ateji

There are kanji less obscure than 論. There are some that are so common that even I know them. It is in the 6th grade jōyō kanji, but I doubt it's in the top 1000 most common characters in Japanese, maybe not even the top 2000. You could probably guess that 侖 is the phonetic, and 論 is a Chinese loanword so that might help, but by itself that's an even less common character, so you probably won't know it if you don't know 論. 侖 occurs as a phonetic in 倫 and 輪, but that's no help, because there it's "rin" instead of "ron". Did anyone guess "rin"?

"Sus" actually is "sub"! As in "look up to". From below, I guess, like the South Wind. Just as an overseer or supervisor sees from above rather than seeing something above him.

The word for lemon, 檸檬, that I mentioned above is a recent ateji. The characters come from some southern Chinese language borrowing the word "lemon" from English. The Mandarin pronunciation follows the characters, resulting in ningmeng, something just different enough that English speakers are unlikely to notice the connection.

The Japanese word for the fruit is also borrowed from English, resulting in the much closer レモン remon. But the spelling is borrowed from Chinese (when kanji are used at all), despite the pronunciation coming from elsewhere.

> There are kanji less obscure than 論. There are some that are so common that even I know them. It is in the 6th grade jōyō kanji, but I doubt it's in the top 1000 most common characters in Japanese, maybe not even the top 2000. You could probably guess that 侖 is the phonetic

It's not an especially common character in Chinese either, though it is part of some common words like 讨论. (论 is the simplified form of 論.) But frequency of occurrence isn't the only way you can fail to be obscure. I call this one of the least obscure characters in existence because it is the name of the most prominent Confucian work, the one conventionally titled Analects in English, 論語 "discussions".

Consider that "spangled" is not a word in common use in the USA, but Americans can be expected to recognize it anyway.

> Did anyone guess "rin"?

I don't remember.

> Presumably when prewett said "When the Japanese imported it, they used the characters much more phonetically," they meant onyomi. (Perhaps they didn't know about kunyomi.) When they say, "but the characters got assigned to the Japanese pronunciation of the word," that is mostly wrong as a description of how Japanese writing uses kanji today

I read prewett very differently than you do. To me, when he says "the characters got assigned to the Japanese pronunciation of the word", he is referring to kunyomi. I don't think that's wrong as a description of how Japanese writing uses kanji today. But I do think it's wrong to describe that use as "much more phonetic" than the Chinese writing system. I see it as much less phonetic.