I believe the term “fertile” is being used as a euphemism for “having a lot of children per family”.
The “ignorant” part is trying to apply the actual definition of “more fertile” to the groups in question -
it’s not correct to say, in actual definition, that immigrant women are “more fertile” than American women, when the definition is “ABLE to have more children”.
Any highly developed society or culture inevitably has a lower birthrate out of choice, whilst a 3rd world (where immigrants come from) culture will have a higher birthrate.
BTW, yours is an absolutely wonderful home page.