I've done some digging. Here is the story. When Hoke took over at SDSU in 2008, SDSU was in the dumps. They were bottom of the barrel in the Mountain West. Hoke apparently recruited really well and built that program up to respectability. He was recognized for the rebuild, and that is how he landed at Michigan. When Hoke left for Michigan his DC Rocky Long took over as head coach at SDSU. Rocky Long won the MWC 5 of the next 9 years. Rocky Long retired in 2019, and he left one of the best defenses in the country with him. Hoke replaced Long at SDSU in 2020, and Hoke brought Mattix with him because Mattix had coached under Hoke at Ball State and later at Michigan. So basically you've got one guy in the Hoke coaching tree replacing another. Hoke laid the foundation for a MWC dynasty of sorts in 2008-2009, and things came back full circle in 2020 when Hoke resumed his previous title as head coach at SDSU.
Rocky Long is now the DC for one of his former assistants Danny Gonzalez at University of New Mexico. Rocky Long's DC in 2019, Zach Arnett, took a position at MSU in 2020 with Mike Leach.
So Mattix is pretty much Brady Hoke's protege. He picked up 4 years of experience as a DC at Eastern Kentucky, where he was pretty successful, and then he rejoined Hoke in 2020 at SDSU. So he will have 6 years total of DC experience after this year.
Also yes it seems like Mattix runs a 3-3-5.
Aztecs' vaunted 3-3-5 defense now in the hands of defensive coordinator Kurt Mattix - The San Diego Union-Tribune (sandiegouniontribune.com)