The "ell" version predates the "mile" version, and at one time was more common. It is not incorrect, but it is rarely heard today, probably because ells are no longer used, so few people know what an ell is.
Thank you very much, Blue Jay! I thought it was a mistake, so it's an old-fashioned option of this idiom. Besides, now I can explain why I've never heard of "ell". PS. I've just noticed that I misspelled a word in my message. It should have been: "Some dictionaries offer..."