I.B. Horner's translation is more thought provoking than the Ven. Bodhi's, but I can understand the justification for his rendering. I just wonder why the Buddha felt an arahant's reflection worthy of comment.
The Nigantha Nataputta claimed that he had permanent knowledge of his wisdom at all times, iirc, and the Buddha was likely taking pains to explain a difference between that claim and his own experience.
On second thoughts, I know that BB significantly updated the Nanamoli/Bodhi MN translation with new editions. My (third) edition looks the same as the quote above. Can someone with an earlier edition comment?
As daverupa says, the point here seems to be that an arahant (or the Buddha) only knows something when s/he turns her/his mind to it. This point is made in other suttas. See this discussion The Buddha's Omniscience. http://www.dhammawheel.com/viewtopic.php?f=16&t=132
You and daverupa may be right; but I still think there may be something conveyed in this sutta that is not readily appreciated, sort of a glimpse into the nature of an arahant's experience. Reflection seems to be the litmus test that confirms whether or not we're liberated from the belief in a self.