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.

[0]: https://www.merriam-webster.com/dictionary/detest