A reactionary is someone who believes in essentialism: that people are identified and wholly defined by a static core character of what they are, that all their behavior is all dictated by this, that the dynamics of humanity are fixed and inevitably fated to unravel in a predictable fashion.
This is the implication here:
An Israeli votes like an Israeli and a Palestinian like a Palestinian, because everyone in the category is the same. Palestinians have higher birth rates than Israelis because this is an inherent parameter of people in each group. The interests of one person are the interests of the ethnic group as a whole.