Perhaps he has access to statistics that haven’t been released yet. Apparently there will be reports in the future which give more specific information:
This from the New York Times:
“The report did not specify which types of crimes the foreign-born inmates were convicted of, but senior administration officials said future reports would include such information. In the meantime, they pointed reporters to data from the United States Sentencing Commission, which they said indicated that noncitizens were convicted of offenses related to immigration, money laundering and drugs at a disproportionate rate along the southern border. Drug-related prosecutions tend to cluster there because of trafficking activity involving the Mexican cartels.”