It strikes me as even more tenuous than that because you could just as easily say “ah well there were no zodiac murders post-D&D, so obviously role playing slakes rather than inflames the psychopathic mind” or something.
It's interesting that if this Doerr theory is true, that means Zodiac really did resemble pop cultural conceptions of him and other Zodiac-inspired fictional killers: a geek, interested in nerdy fantasy hobbies once considered the province of social misfits. But then again, the occult-steeped tone of the letters and the ciphers themselves already gave that impression.