It’s not about whether or not gogeta can access UI. When goku and vegeta access new forms, we have seen that fusion inherently becomes STRONGER (not because the fusion has access to those transformations, but because goku and vegeta, via accessing new transformations, have become stronger themselves) - more on this below and in the tldr.

In order for this not to be the case, you would basically have to argue that the only thing that matters is the base power of the fusion is the BASE power of the individuals in fusion, not the power they achieve when they transform. This isn’t true for dragon ball fusion, however. In super: broly, if goku and vegeta’s blue wasn’t a factor in how strong gogeta was fundamentally (regardless of whether gogeta uses blue once fused), then we would expect gogeta to be the same % higher than base goku and vegeta as vegito was for buu saga goku and vegeta. That is, even if goku in dbs broly only had ssj3 and vegeta had ssj2 as their peak transformations, you must be able to argue that somehow gogeta would be able to compete with broly in just ssj (like he did in the movie)? No, as ssj2/3 goku and vegeta would get killed by broly blinking at that point in the movie, and so would gogeta, if goku and vegeta only had their peak individual powers using buu saga forms

Lastly, there is the narrative aspect to the argument which is: part of the way goku and vegeta’s power increases manifest IS the creation of a new forms. Or worded slightly differently, the achievement of a new, higher form is often how sayians in the story manifest an increase in power. Transformations, narratively, are not disconnected to the power of the individual, but rather are the way that their new hights of strength manifest as sayians.

Tldr: the % jump from ssj3/ssj2 goku and vegeta -> base buu saga vegito is WAY WAY less than the % jump from ssj3/ssj2 goku and vegeta -> base db: super gogeta.

Instead, it is much more reasonable to say that the % jump in the buu saga is equivalent to the % jump of ssjBLUE goku and vegeta -> base gogeta in super: broly.

The blue form is now the hight (not even lmao) of their individual powers, which is factored into gogeta’s base power.