They’re limited in the sense that they don’t have the ability to advocate for themselves in a court of law or in the court of public opinion. They’re entirely dependent on others to advocate for them. That’s what makes them different from a typical adult human.

Compared to healthy, functioning adults how are infants not limited?

Person is a legal concept, too.

Of course infants and intellectually incapable adults are limited persons in the legal sense: they have fewer duties, but also fewer options. For example, they won't be accepted to a pilot school.

This could be turned round - are you implicitly assuming a human with limited ability has less value? Mere skill isn't the same as being a "good person", after all.

No, not at all. I'm a Christian and it's dogma for me that all humans carry the image of God and have intrinsic worth independent of their abilities or utility.

It's not about a "superiority complex" but about limited responsibility and independence.

99.9999% of natural resources in the universe are atoms of hydrogen in stars. We can't say that those atoms of hydrogen are a person, what we are really saying is that our probable intention to "exploit" those atoms of hydrogen makes us a vermin. I'm, of course, terrible opposed to that notion.