I'm not so sure that global population growth tells the right story vis-à-vis declining birth rates in western countries.
Well, they were directly asked what they meant and replied with a link to a giant Wikipedia article. If they had a more specific point to make, they're being weirdly cagey about it.
As long as everybody is nationalist (not to say racist) and keeps borders closed to protect the homeland, those differences matter. Realizing that one thing people get worked up about (immigration) is a solution to the other thing they get worked up about (not enough kids!!1) would be a great insight.