I was wondering precisely the same thing. Looking closely at the inscription, it appears to have been made by using a pointed instrument with green color at its tip, applied one dot at a time.
But in general, if you hit an iron rod with another pointy iron rod, you'll probably leave a mark. Do it many times, and you have an inscription. Probably took a lot of work though, so the guy who owned it probably was quite busy for some time... Maybe a long road from Rome to London :)
The article says it is made out of iron, but wrought iron is much more likely. The Romans would have been able to make a punch out of hardened wrought iron to mark this stylus.
I'd bet on the "added now", possibly by 'shopping the image ...