It wasn't clear to me at first but I get it now. The question raymondaliasapollyon was asking was whether the antecedent of it is land or any land. What role, if any, does the determiner any play in the reference? Is the reference to the whole noun phrase (any land) or only to the head (land), to the exclusion of (i.e., excluding) the determiner.
By my analysis, (which is purely pragmatic) it doesn't really matter. What matters is what Walton had in mind when he uttered the word it, which was the piece of land that was closest to them, upon which the monster was travelling, whether that piece of land was only imagined or not. The monster could have been riding on ice, yes, (in fact it seems he was) but the idea is that Walton's reaction to the apparition was that it must have been terra firma, or at least it seemed that way to him at the time.