I think we got a bit sidetracked; if I am not mistaken, this whole debate about Gollum came up only because he came up as proof of Hobbit-toughness. And that came up only as a side-remark about something WK might have thought of, and
that came up only in relation to yet another side-remark that the Nazgūl may have been surprised by Frodo's resistance.
And that all being said, the entire argument stands on the assumption that the Nazgūl knew that Gollum was the same type of creature as the Hobbits. I am sort of wondering whether it might not have been beneath their discernment capabilities.