It's not the upper bound that matters but the frequency. How frequently should the compiler assume an 's' appears in the dataset, or any other character?
We know that E[# of '\0' in a string] == 1.
But what is E[# of 's' in a string]? Is it greater or less than E[# of '\0' in a string], and how should the compiler know this?
You haven't given the compiler any reason to assume that 's' or 'p' will appear more often than '\0'.
Imagine a scenario where most of the strings being processed contain a single null character, with no other characters. In that case checking for the null character first would be optimal.
Does the compiler know that this isn't true? No, it doesn't. The author of the article is making an assumption about the contents of the data that might seem reasonable but isn't necessarily true.
We know that E[# of '\0' in a string] == 1.
But what is E[# of 's' in a string]? Is it greater or less than E[# of '\0' in a string], and how should the compiler know this?
You haven't given the compiler any reason to assume that 's' or 'p' will appear more often than '\0'.