Herbivore men in Japanese is not "soshoku danshi", it's "soshoku KEI danshi". Just sayin'.
You are correct. The calligraphist got the inclusion of the "kei" right, just not the article text. I don't think that "soushoku danshi" is wrong, though. Also, sometimes just "soushoku doubutsu" is used (literally, "herbivore").
It's not even a word denoting vegetarians, but rather herbivores. A vegetarian is called 菜食主義者 (saishoku shugisha), not "soushoku" anything. "soushoku danshi" comes from "soushoku doubutsu", or "herbivore".