You are probably right. “and subject to the laws thereof” is the phrase in the 14th Amendment.
The law would offer the definition that children without at least one US citizen parent would not be subject to US law when their parents returned to their home of citizenship, and so such a child born in the US would not be eligible for birthright citizenship.
The law could, (note I do not say ‘should’) even make the same definition for children without two US citizen parents.
Currently in Title 18 of the US code there is a provision that children whose parentage is unknown and who is raised in the US from prior to the age of 5 years is be awarded US citizenship.