In fact it need not even be restricted to discrete signals. I think he was uncomfortable with the fact that the continuous signal story was not as well fleshed

In fact it need not even be restricted to discrete signals. I think he was uncomfortable with the fact that the continuous signal story was not as well fleshed out. He came to revisit that after about a decade in "coding theorems for a discrete source with a fidelity criterion". This did for lossy compression what his 1948 paper did for lossless. It rarely happens that a paper that creates a field also resolves its most important questions. That was Shannon's standard, I guess. That he revisited the question perhaps speaks to his inner discomfort that the story was not complete.