Remember "'His cat,' he says, though she owns him not?" Or "I pray the Lord my soul to keep?" Tolkien loves inversions and old phrasings, and Gandalf may well be saying "The Nine keep the Nazgul."
More likely, he never noticed or corrected this statement after he had thought more about the problem. Sauron after he lost the One simply couldn't exercise dominion over the Nazgul if they still had their Rings: they would have been free agents.
The most extensive discussion of the matter is found in Letter #246, which in part discusses what would have happened at the Sammath Naur without Gollum's intervention.
Quote:
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Sauron sent at once the Ringwraiths. They were naturally fully instructed, and in no way deceived as to the real lordship of the Ring....they would have obeyed or feigned to obey any minor commands of [Frodo's] that did not interfere with their errand - laid upon them by Sauron, who still through their nine rings (which he held) had primary control of their wills.
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I doubt it gets more explicit than that.