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.
But because in the single-null case the loop body is executed only once, the gains of arrangement that prefers nulls are pretty slim compared to long-string cases where the loop body is executed many times. For example if your dataset contains 99 cases of single null strings and one case of 100 chars long string, it might still be optimal on aggregate to use the long-string optimizing arrangement.
Of course there are still some cases where non-zero strings are extremely rare and as such optimizing for those makes sense.
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.