There was actually a study shared on a thread here that showed BSSO itself actually does physically increase bi-gonial width in a large majority of cases. The effect was greatest immediately post-op and attenuated over the following year, but the 'stable' width was still greater than baseline. The increase in BGW was not associated with the size of the movement.
Here's an example from Movahed (amazing result IMO) where real/perceived width increased in a similar way as the case you posted:
https://movahedoms.com/services/osa/case-study-4/
Maybe it’s a little objectively wider from the BSSO alone. I didn’t know it objectively widened the jaw. So that does play a role.
However in the case you posted I think it’s mostly is the perceptional change from the reduced mandibular plane angle. It’s less steep and downwards at the sides and more forward, giving an improved GA and the illusion of a wider jaw... but it’s more like a well-balanced or harmonious jaw.
For example someone with a long and pointy chin is going to appear to have a narrower jaw than someone with a shorter chin, even if the GW is identical in both cases.
When I look JUST at the goinals of the Before and after I’m hard-pressed to notice widening. I’m sure it’s there, it’s just not too obvious.
When you step back and perceive the face as a whole and how the parts relate, the improved goinal angle seems to account more for the perceived facial width increase.