He doesn't, and no one said he did.
If it is the case that he’s getting something from the UNT connected with his past career as a state jurist,
He never worked as a judge for the state. Look up his bio. He did work as an assistant county attorney in Texas, but that may have been on a county payroll, not the state.
He gets a full federal pension from the United States and he gets salary from the state of Texas. There is a loophole for federal judges where they can receive a full federal pension and receive a 2nd salary as long as the 2nd salary is in the field of education.