I gather the 27 words fro snow is a myth. Pity. But in any case, as I understand it, the thing about pure sign language is that it isn't words as such that are being signed (and I know that the sign language people use are in fact full of words).

In a way I'd imagine that pure sign language would be more analogous to Chinese ideograms, where the same ideogram can be read as totally different words in different varieties of Chinese, but have the same meaning.