adjectives - "more" is to "less" as "er" is to what?

Excerpt from Cambridge Dictionary of American English:

If you want to use an adjective or adverb to say that a quality is of
a higher degree, you can usually add -er (one-syllable adjectives) to the end of it or qualify it with more (adjectives of two ore more syllables).

e.g. your hair is longer now than it was last year.

To say that a quality is of a lower degree, you can usually add
-er (one-syllable adjectives) to the end of a negative adjective or adverb, or qualify it with less (adjectives of two ore more syllables).

e.g. your hair is shorter now than it was last year.

My question is: How can we say that a negative & one-syllable adjective is of a higher degree? I mean, if "shorter" somehow means "more short", how can I say that something is "less short"? And if "longer" somehow means "more long", how can I say that something is "less long" than another thing?

A friend of mine suggested that the only way of saying the opposite of "negative adj + er" is to say "positive adj + er". (i.e. shorter -> longer). My take is that whether "adj + er" means "more adj" or "less adj", entirely depends on whether the used adj. is positive or negative respectively. Is my understanding correct? Is it possible to say the opposite of "shorter" to convey the meaning of "less short" without using a different adjective?

Rephrased question:

For non-one-syllable adjectives:

If A is more beautiful than B, then B is less beautiful than

A.

Why is there no such ability in English to bidirectionally compare one-syllable adjectives as well?

If A is rounder than B, then B is (???) than A.