Thats what the law says.
An infant is a child who is already born. Actually, legally, it referred to minor children as well as what we today would call infants.
In other words, "infants" were not necessarily sitting in a rocker or crawling on the floor.
Did you not even read the part of the law that said all persons born in Virginia were citizens?
I dealt specifically with the citizenship of minor children, which according to the law follow the citizenship of the parents.
the father - or if he is dead, the mother - are citizens;
OR
the father - or if he is dead, the mother - become citizens;
OR
migrate hither without father or mother
Except for foundlings which become citizens, the citizenship of the minor child follows that of the parents.
"and all others not being citizens of any the United States of America shall be deemed aliens"
Just like US Naturalization Acts, just like the Penn. Supreme Court, just like the cases I've cited.