I think this poster is a nutcase, but he’s obviously an older nutcase and probably from California or someplace in the West. Young people wouldn’t know about the Zodiac Killer, and somebody from the East would think of the Mad Bomber first.
The Zodiac Killer has been referenced in a number of modern TV shows, etc. for young people to know about.
You’re assumption sounds like the correct one.
Recently there was a mini series, on TV, investigating the Zodiac Killer.