Ummm no it isn't, says so right on the sharpie. "Nontoxic, conforms to ASTM D-4236. Made in the USA" Wonder what is coming off the cheap Chinese ruler.
Read a little closer: "They are considered non-toxic
for "normal uses", meaning writing on posters, soccer balls and such." Not ingesting! Sharpies contain n-propanol, n-butanol, diacetone alcohol and cresol. n-propanol, is commonly used in cosmetics, but the other three are industrial solvents - chemicals that should not be sniffed, eaten, or put on the skin.
And the ruler is momentarily dipped in the cool water to gauge strike/sparge water volume before heating. Probably not much coming off of it into the cool water in a couple of seconds. The Sharpie ink, however, is in contact with boiling wort for at least an hour.
Beernut, I apologize, I'm not trying to be a jerk about this or anything. It's your beer, you can do whatever you want with it. I'm just saying personally, I wouldn't risk it - not when there are other ways to accomplish the same thing, that don't require exposing my beer to potentially toxic solvents. I just use a wooden dowel that's been notched to indicate gallon marks.