Clearly, what he and Garaki did for Number 6 aided in pushing Number 6 beyond the point of singularity.
Rewriting Number 6's biology and replacing normal tissues with Bomber Cells…
- …bypassed the natural bodily Quirk restrictions, allowing for a total of five Quirks—only one less than the High-End Hood;
- …enabled a degree of shapeshifting which rivaled the likes of the Nomu's Muscle Augmentation, Transforming Arms, Liquification, and Storage;
- …and granted enhanced physical abilities rivalling the likes of Knuckleduster and Octoid, whose own abilities are Pro Hero-level.
Kuin only distributed Trigger and then studied the effects. Evidently, though, given the various specimens All For One would later acquire in the pursuit of mass-producing Nomu after her death, her role was relatively insignificant in the grand scheme of things.
Number 6 is allowed "to act as [he sees] fit", but the operations are spearheaded by All For One—part of his agenda to surpass All Might. He provided Number 6 with his Quirk, his identity and mentor figure, and his resources, like the Bombers and the headquarters of the factory itself.
The point is: he had a perfect framework for the Nomu and then completely disregarded said foundation and started from the top again? Like, Kurogiri was an intelligent Nomu, Number 6 and Gigantomachia were effectively perfected Nomu, and Hood had been completed and actively in-testing during All Might's Silver Age. And his key takeaway from all of his previously relatively successful projects was to create mindless brutes with weaker stats and fewer Quirks?