BP, I checked the index of Tolkien and The Silmarillion, Clyde Kilby's enjoyable little book about his summer with Tolkien, etc., and find no mention of Dunsany. Rateliff is a first-rate Tolkien scholar, so I'll be careful about assuming he's made a mistake. Possibly he had private communication with Prof. Kilby and the latter told him about the incident. On the other hand, on one occasion at least I've thought Rateliff floated a possibility that was unlikely to be correct (that Inkling Charles Williams was influenced by Lovecraft). But there, Rateliff does not state something as fact, just as possibility. So for the moment I'll guess Rateliff's account is correct. It is believable that, very early on, as Tolkien was creating "a world for the languages," he was indeed influenced by Dunsany. But the mature Tolkien is about as un-Dunsanian a fantasist as there could be.