The main quote comes from Ethan Pettit: "Evan Thies has not only taken the lead in the fight to stop Atlantic Yards, his 360° Planning Initiative will stop the next irresponsible megadevelopment from getting off the ground."
Pettit may believe that, but that doesn't make him an authority. A search on ("Ethan Pettit" AND "Atlantic Yards") shows he's had no public presence on AY other than signing a petition on Thies's web site.
Thies has expressed strong opposition to AY, but I wouldn't say he's taken the lead. For example, he didn't testify at: 1) the public hearing held in July by the Empire State Development Corporation (ESDC); 2) the state Senate oversight hearing held in May; 3) the Metropolitan Transportation Authority's two public meetings in June; and 4) the ESDC's public meeting in June.
The bottom of the mailer quotes this blog: "Thies has probably the most extensive set of suggestions" regarding development. That comes from a piece in April in which I noted that, while Thies was expressing strong opposition, he hadn't aligned himself with Develop Don't Destroy Brooklyn, which has taken the lead in opposing Atlantic Yards.