This overstates things. There are many, many, theoretical physicists who worked on regular old quantum field theory in both a fundamental and material science vein. I'd guess that if you got all the theoretical physicists in the world together, string theorists would make up less than 30%.
I hope you mean all the theoretical physicists working on the high energy theories of physics, because 30% is a sadly high figure even for that subfield. If 30% of all theorists would actually be of no use to any other physicists, that would be horrendous.
