Unless the article is wrong, the authors of the study didn't actually track people that were paid low wages before and after the minimum wage increase.

Instead, they just looked at the number of hours worked for under $19/hour before and after the wage increases. Since $19 is close to the new minimum wage, it could be that the pay scales of everyone shifted up a bit, and jobs below $19 are now a lower percentage of all jobs.

Unless I'm missing something, this study is too sloppy to be newsworthy.