I don't know, really. That's just what Professor Tolkien wrote in the third part of Notes on motives in the Silmarillion. It's hard to say whether it was "true" or if it was just what Professor Tolkien considered as a rationalisation for the narrative choices he had made.
(This is sort of intended to respond to denethorthefirst as well.)
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"Since the evening of that day we have journeyed from the shadow of Tol Brandir."
"On foot?" cried Éomer.
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