They are, but the child is not. The Child is subject to the jurisdiction of the United States. The Constitution says the person, not their parents, must be "subject to the jurisdiction". The only children born in the US who are not fully subject to the jurisdiction of the US are those covered by agreements with other countries, such as the children of ambassadors, military personnel, and some other expatriates.
But just because a child is a citizen, does not mean that he/she must be an "Anchor Baby". The status only accrues to the child, not to the parents.
The fact of being born on US soil does not automatically make the child a citizen. If it did there would be no need to add the "and subject to..." qualifier.