Maybe you can look at the tables and find where it gives a breakdown of crimes by illegal immigrants vs non-illegal immigrants.
There doesn't seem to be one so the author is pulling it from somewhere.
There's citizens vs non-citizens but that's a much different statistic
The article states that
"The United States Sentencing Commission has released showing that almost three-quarters of the more than 2,200 people who received federal sentences for drug possession in fiscal year 2014 were illegal immigrants. Moreover, illegal immigrants were more than one-third of all federal sentences for all crimes."
However the table (table 9) is citizen vs non-citizen and why is that the important criteria when almost 75% of drug Trafficking and 82% of Drugs - Communication Facility (whatever that is) is down to US citizens?
Also note that 20,823 of 31,432 crimes by non-US citizens are Immigration Offences...
So while it's true that over 1/3 of all federal sentences were for non-us citizens about 2/3 of the crimes by non-us citizens are migration offences