They had his DNA from the back of stamped envelopes he sent in the early 1970’s.
From the [yet to be updated] Wikipedia page:
In May 2018, the Vallejo Police Department announced their intention to attempt to collect the Zodiac Killer’s DNA from the back of stamps he used during his correspondence. The analysis, by a private laboratory, was expected to check the DNA against GEDmatch.[88][89] It was hoped the Zodiac Killer may be caught in a similar fashion to the “Golden State Killer” Joseph James DeAngelo. In May 2018, a Vallejo police detective said that results were expected in several weeks. However, as of September 2021, no results have been reported.[90][91][92]
And yet, the article didn’t say HOW the came to this conclusion.
Must mean there’s a true crime documentary in the works...