Yes, IEX has been an ATS for years, but that is completely different than their new primary listing exchange. Historically, the US had multiple primary Exchanges operating at one time, but now they are all gone except for the 3 owned by the NYSE and NASDAQ.
It will be interesting to see if they get much traction. With the high percentage of trading now done "off-Exchange" trading done via ATS, the primary trading venue is not as important as it once was, and that may work in their favor.
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