If you're talking about Mr. Leiterman, no, it was never proven (or for that matter, disproven) that he was in the area of where either Jane Mixer was last seen or where her body was found.
In 1969, Gary Leiterman was living near Ann Arbor, however, where he spent much of his younger years. He was born in or very near Detroit in 1943. The other man in question in this whole case with the DNA, John Ruelas, was born in 1965 and at the time of the Mixer homicide, was living about forty miles away from Ann Arbor. There is absolutely no evidence that Ruelas, his family and Leiterman knew each other at any point before the trial in 2005. (Nor did any evidence ever come up that Mixer, or her family, friends, knew either Leiterman, Ruelas, or their known associates).
The fact that both Leiterman's and Ruelas's DNA samples were both being processed in the lab at the same time as Mixer's evidence was points towards possible (well, likely) contamination.
I mean, what would really be the odds that murder victim and killer(s) both happened to have their DNA samples be processed at the lab during the same week---thirty-three years after the fact?? That would be incredible. The "Gary is guilty" (or Ruelas is involved, too) supporters seem to miss this very crucial point. The chances of both, let alone all three, coming into the lab the same week to be processed would be well, to put it lightly, unbelievable.