The article does say that despite using the Safe Sea, some people will still be more affected than others.
Are you very sure you applied to the area where you have the ring? For example, I burned my feet terribly in Hawaii one time. All because I hadn't slathered suntan lotion all the way to my toes. It was quite an interesting burn pattern.
I just got back from Roatan last week as well and also didn't have any Safe Sea but didn't get stung either. However, I was diving with a couple that were absolutely covered all over their cores. They had encountered the pica pica two days before I met them. Wicked looking. I didn't find out if they had been wearing wetsuits or not when stung like that.
What side of the island were you diving from?