In "ein weiter Flug", "weiter" is an adjective, and "überlegen" is an adverb. The adverb is used attributively to modify the adjective, and in this case it does not make sense for the adjective to be in comparative form. Also, in this sentence there's nothing to compare the adverb itself (superior in quality to what?) against, so the adverb can't be in comparative form, either.
Therefore, "ein überlegener weiter Flug" (where "überlegen" is meant to be an adverb, not an adjective), "ein überlegen weiterer Flug", and "ein überlegener weiterer Flug" (s.a.) are all out. (One could use "überlegener" as an adjectiv on its own, though).
It works the same way in English: You'd say "an exceptionally long jump", not "a more exceptionally longer jump".
In principle it's possible to use the adjective on its own in comparative form ("ein höherer Sprung"), but "weiterer" also has the meaning "a further, furthermore, another", so "ein weiterer Flug" could be misunderstood as "noch ein Flug". I guess this is why the author choose "überlegen weiter" instead.