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How might he "not have left DNA behind" when the charge was rape! And DNA evidence was used to used to clear him.

According to the article:

In October 1977 a Memphis woman was raped in her home by two intruders...

I don't think we need to get into specifics, but you don't need to ejaculate for it to be considered rape.

It's not like they have a time machine to go pick up old evidence. They had an old sheet, it's easy to imagine how there wouldn't be DNA left there.