The label is a bit weird, I agree, but personhood is an established legal concept with clear interpretations and case law so I just think of it as lawyer speak. Extremely disabled people may lack clear communication and yet a person who makes decisions on their behalf is guided by similar principles. The argument in favour of personhood is that the benefits of that legal history outweigh the superficial qualms by using a slightly wrong title.

While other animals have sentience too this is about the thin edge of the wedge, and what's achievable in politics. You go for the creatures that are nearest to human and then increase it as is appropriate. It's important to establish precedent and not to go too far at once or else the first step will fail and politicians won't touch it again for a long time. I see it more as practical political expediency rather than a lack of understanding of how non-apes may be sentient too.