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Why use ‘still more’ when the sentence’s grammatical form is positive, but ‘still less’ when negative? 

i quote https://usage_en.en-academic.com/3072/still_less 

>>The principles, much more the practice, need a good deal of scrutiny. I didn't even see him, still less talk to him.
>
>*Much more* (or *still more*) is used when the grammatical form of the sentence is positive, and *much less* (or *still less*) when it is negative. Uncertainty arises when the form is positive but the sense is negative, as with adjectives in *un-* and words like *difficult*. In the sentence
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>>It is difficult to establish all the facts, much less to reach a conclusion
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>*much more* is strictly needed, not *much less*, but the result is awkward and an alternative such as *let alone* is often preferred.]

How can Syntactic Theory explain [the paragraph above](https://usage_en.en-academic.com/3072/still_less)? Why must *still more* be ‘used, when the grammatical form of the sentence is positive’ and *still less* when negative? 

Why can’t *still less* work for both + and ― grammatical forms? 

Why can’t we swap *still more* and *still less* in the above paragraph, so that we use *still less* when the sentence’s grammatical form is +, and “still more* when ―? 

This quirk happens in French too.

>>(Il est/C'est) difficile de pardonner, et encore **plus** d'oublier.
>
>[The usual meaning of 'let alone' is "et encore moins", but the actual sentence content makes it almost mandatory to turn this upside-down - hence my suggestion.](https://forum.wordreference.com/threads/let-alone.1097186/#post-11726253)