But this is not really much to support sutta < sūkta, since the regular Pāli form parallel to sūkta includes the glide -v-, as Skt ukta ~ Pāli vutta and similarly in other MIA languages, which all seem to preserve the initial v- of the verbal root *vac- (Pischel §337), despite the vowel change a > u before a labial (§104).

And anyway, Buddhaghosa here is offering multiple exegetical “etymologies” (an old technique beginning already in the Vedic brāhmaṇa-prose), which are alternative or mutually complementary.  The last of the six offered here relies on the “thread” meaning, explained using two distinct analogies which, if anything  about the author’s sense of the basic literal meaning of the term is to be inferred from that fact, would point rather to a stronger awareness of sutta as connected with threads:

… suttasabhāgañ c’etaṃ yathā hi tacchakānaṃ suttaṃ pamāṇaṃ hoti evaṃ etam pi viññūnaṃ, yathā ca suttena saṅgahītāni pupphāni na vikirīyanti na viddhaṃsiyanti evam etena saṅgahītā atthā.

The trans. of the whole passage:

This Scripture shows, expresses, fructifies,

Yields, guards the Good, and is unto the wise

A plumb-line; therefore Sutta is its name.

For it shows what is good for the good of self and others.

It is well expressed to suit the wishes of the audience. It has

been said that it fructifies the Good, as crops fructify their

fruit; that it yields the Good as a cow yields milk; and that

it well protects and guards the Good. It is a measure to the

wise as the plumb-line is to carpenters. And just as flowers

strung together are not scattered nor destroyed, so the Good

strung together by it does not perish. Hence it has been said,

to facilitate the study of the word-definition:

This Scripture shows, expresses, fructifies,

Yields, guards the Good, and is unto the wise

A plumb-line; therefore Sutta is its name. 

(tr. Maung Tin, The Expositor, v. 1, PTE (1920), p. 24

Best,
Tim