They just posted the article for the Rune Golems expansion. https://www.fantasyflightgames.com/en/news/2017/7/13/stone-sentries/
In the article they once again say something about the attack modifier adding a rune:
" This is complemented by the golem’s command tool modifier that allows them to add one stable energy when participating in a skill or melee attack."
Did they make this mistake again or is this really what the modifier is supposed to do?