Author here. My intent was to say, "When someone I know asks me if RL is a good idea, the answer is no >= 70% of the time." The important missing piece is that it's conditioned on that person asking me in particular. I don't have the largest profile, so the people who ask me about RL are generally well-informed about RL and have some practical ML experience too. Within that set, 70% felt like the right lower bound.