I may be reading too much into this, but I have a hunch that IEX has made Citadel angry:
Citadel is, of course, one of those "market makers that execute most retail orders," so it has a dog in this fight: Retail brokers send most of their customers' orders to "wholesale" market makers like Citadel, unless the customers specifically direct the brokers to send the orders elsewhere. Citadel's comment letter to the Securities and Exchange Commission on IEX's application to become a public stock exchange goes on in that vein, for five pages. "IEX behaves as though it is entitled to special treatment because its owners were lionized in a popular book by a celebrity writer," says Citadel. It is a fun read!