I expect it was based on maps found in the Ambarkanta, in The Shaping of Middle Earth (HoME IV). Hildorien is shown on these maps. These were the only complete maps of Middle Earth Tolkien ever drew, and they are very sketchy. Personally, I wouldn't like to think of them as 'Canon'. They were made before the Lord of the Rings was even begun, so Tolkien almost certainly would have changed his mind about a great many things.
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"If you would be a real seeker after truth, you must at least once in your life doubt, as far as possible, all things." -- René Descartes
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