Ameritrade JUST confirmed this with me too--you cannot go any lower than 4 digits out for limit orders.
BUT, Ameritrade DID confirm that MM's have the ability to fill your orders at 5 digits out.
If Diamond filled his limit order at 5 places out (especially at .00015)--then it would still be reported as filled at 4 digits out. (Correct me if I am wrong there--but this is what TD Ameritrade told me).
From my understanding, MM's typically trade at that price point if multiple orders come over at the same time on the tape.
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