The findings in the studies that are cited in the article depend on two factors. The first is how to define unskilled workers. Some researchers include both high-school graduates and dropouts. In 2014, there were 64m such workers aged between 25 and 64 in America. Others prefer to treat high-school dropouts separately in their research, so that the lowest-skilled migrants compete with fewer existing workers: 20m, at last count.
The second factor is whether, among those with similar education, migrants and native workers are substitutes or complements for each other. Some studies found that immigrants seem to compete mostly with other immigrants, even when controlling for age and education.
The article continues to say that the flipside of low wages for illegal immigrants, though, is greater economic benefits for those who are not competing with them for work. A rare study of the effect of illegal immigrants specifically found that in Georgia, a one-percentage-point increase in undocumented workers in firms boosted wages by about 0.1%.
The debate continues.