They checked voter records against driver's license records. If someone used non-citizen documents to apply for a driver's license they are being tagged as non-citizens in voter registration records. The problem, of course, is that individuals who are Texas residents but not US citizens at the time of their application for a driver's license can and do change their citizenship status. Further, there is no requirement and no reason at all to notify the department of motor vehicles about said change. Therefore, there are LOTS of US citizens who used non-citizen documents to apply for their driver's license.Dude, they're not citizens. No legal ID will legally allow them to register to vote if it's known they are not citizens, whether they are here legally or not. So either they fraudulently registered, or government workers allowed them to register, which is also illegal. Not one of those non-citizen voters had a legal right to do so.
To provide some idea about the magnitude of the problem with this "study", about 50,000 Texas residents become naturalized citizens each year. Over the period of the study (1996 - 2016?) that would be about 1,000,000 Texas residents who have become naturalized US citizens. With this, it is actually surprising that they found only 95,000 discrepancies between driver's license records and voter citizenship status.