It's not something that "the dems" made up. It seems to have started as a self-descriptive term for nonbinary Latinos, although the history's not 100% clear. But as the source article describes, there's a large and influential segment of the populace who believes that in order to be inclusive they should use trans-focused language as much as possible. So people started saying "Latinx" for precisely the same reasons as they stopped saying "pregnant women".