Their brain is literally broken:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3286221/

https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.600.47&rep=rep1&type=pdf

https://www.psychiatrictimes.com/view/neurobiology-borderline-personality-disorder

The therapies they engage in literally train their brain to begin to recognise their internal reactions because they are broken, they literally cannot do self reflection, they need to be taught and even then, it is a mask. Yes some of it is emotional, but what of the dissociation, the manipulation (conscious or not), the outward behaviour that is 'uncontrolled'?

I have had experiences where my pwBPD was screaming blue murder at me about how I was wrong about something and then not 5 minutes later literally said they didn't remember what they just said or how they were screaming. I had to video record it the next time to show them.

The neuropsychological research on BPD is extensive and they pretty much all say the same thing. Parts of their brains are smaller (amygdala, hippocampus  and the orbitofrontal cortex). Their brains are broken.

I am glad your wife got 'better' but Piaget's theories are incredibly limited in their response to cognitive development and discount the many socio-cultural and linguistic influences on behaviour in the development of children and have nothing at all to do with BPD.