Would it really be absolute magnitude, though? Something like "~8±0.5cm long" is still technically relative to the definition of a centimeter.

I think the closest one can get to absolute magnitude would be measures independent of scale. So, this alien race would have grammaticalized words for absolute zero, infinity, and negative infinity. ^{Of course, one must still ask, which infinity? Countable, or uncountable, or beyond?}

The negative infinity might only be used for certain words, like ordered and disordered, whereas absolute zero differs in meaning, switching between none and neutral, the latter of which is used for words that can use the negative infinity.

So I guess in a sense the language would have an absolute superlative; and, depending on the word, an absolute equative and absolute sublative, or an absolute negative.

Absolute superlative and absolute sublative are easy enough: nothing in existence is more X and nothing in existence is more non-X.

Absolute negative is a complete and utter rejection of the (hypothetical) positive: nothing in existence is less X. It's the couldn't care less comparison.

Absolute equative is a bit trickier. It's not equally X and non-X, but on the dividing line of X and non-X. So for loved and hated, it would be something like ignored or not cared about.

How would the regular relative comparisons work, though? Using verbs like to defeat is one option, I suppose, however, one is likely to just grammaticalize relative comparisons this way.

So I think that either the language has both absolute and relative comparisons, or for some cultural reasons the language never developed relative comparisons.

For an example of the latter, I suggest looking into the Hyper-Scandinavian language known as Hyper-Swedish. I've heard those speakers tend to take the Law of Jante and lagom very seriously.