Interesting. I've never heard this use. The definitions I've seen of detest are some form of dislike intensely.
Reviewing Merriam-Webster[0], I see there is an obsolete sense of detest that means to curse.
> 1 : to feel intense and often violent antipathy toward : loathe · detests politics · They seem to truly detest each other.
> 2 obsolete : curse, denounce
I can't recall ever having across across the word used in this way in American English. I've only seen the first sense used, and would have interpret someone saying "despise and detest" to be effectively doubling (redundantly) for emphasis, not to distinguish between the two.
Etymology doesn't determine meaning.
I'm open to the possibility that in 1950s England these words meant what you say (even though in my native dialect of English they don't), but to argue that you would need to give citations, not etymologies.
Words mean things. Just because you, and many others don't always observe the minutae of meaning, does not erase it. There are words you, and 1000 other peers, have not learned yet. That does not render them meaningless.
That's not what he said.
Not for the last 2-3 centuries it is not. For people reading Beowulf in the first edition, maybe.
Detest
1. to feel abhorrence of; hate; dislike intensely. (dictionary.com)
1 to feel intense and often violent antipathy toward : loathe detests politics They seem to truly detest each other. 2 obsolete : curse, denounce (Merriam-Webster)
1. to hate someone or something very much (Cambridge dictionary)
1. Dislike intensely (Oxford dictionary)
>Their roots come directly from Latin/French.
Which is irrelevant as to their current meaning.
Now, we are not that different. My correction is also driven by the same joy of correcting others that makes people learn some obscure obsolete factoid and insist on it as if the whole world is wrong and they are right. The only difference is I'm right :-)
"Detest" and "despise" are not synonyms. To detest is to speak out against; to despise is to look down upon. Their roots come directly from Latin/French. If you detest something, you are issuing speech which indicates that you dislike it. If you despise something, you have observed it and disliked your observation (or, more usually, something about the detail of the observation!)
When people use these words in idioms like, "I despise and detest their actions," they are saying that they have seen the actions, they do not like what they have seen, and they are now speaking out to denounce those actions.