Numerical example of theoretical diff file size using Kullback Leibler?
I understand that the theoretical size of a diff patch between two similar files can be calculated using Kullback Leibler (KL)


That's not quite right.  I suspect you've taken an imprecise, non-technical, handwavy statement from Wikipedia and interpreted too much into it.  They're trying to summarize some underlying theory in an informal way, but such summaries are inevitably incomplete.  Instead, you'd be better off reading and understanding the underlying theory.

There is a sense in which that statement is correct if we make certain assumptions -- e.g., that the source is memoryless (each character is independent of all past ones, with no history), and that we interpret it in an asymptotic, average-case kind of way -- but for many real-world sources of  data, those assumptions don't apply.