You raise a point. But, in my own opinion, every single shadow isn't exactly the same and not everyone's shadow is 100% negative. Sure, the P4 shadows were just a bunch of distorted exaggerations about their biggest insecurities, but a Jung shadow in itself is not just their darkest parts. A Jung shadow is what they reject or are ignorant of. It's probably not something you're proud of or want to be aware of but it's not Gritty McEdgey. But I don't really know. I personally thought it fit; Sato doesn't deny much about himself, and it's really just the biggest part that pushes him over that edge of denial. It's that last part that holds him back. But again, that's just my opinion.
Regardless, I had nothing better, I liked it, and it had slight relevance so I just decided, "Why not?"