"theft" is so incorrect that you damage the credibility of your argument by using the term.
You have a social/emotional complaint: someone took your good idea (and kudos to you: it's a great idea!), and duplicated it. But it's recognized (and documented here) that you were the progenitor of the idea, and if you eventually open source the original, I can't see how this will "hurt" you. People have an innate sense of fairness, and duplicating ideas like this goes against it in a small way.
But it's not theft (or even copyright infringement), and by overreacting you are going to alienate people who would otherwise be sympathetic.
You have a social/emotional complaint: someone took your good idea (and kudos to you: it's a great idea!), and duplicated it. But it's recognized (and documented here) that you were the progenitor of the idea, and if you eventually open source the original, I can't see how this will "hurt" you. People have an innate sense of fairness, and duplicating ideas like this goes against it in a small way.
But it's not theft (or even copyright infringement), and by overreacting you are going to alienate people who would otherwise be sympathetic.