No the author is saying the opposite. They expect file9.txt to be after file10.txt, but it many modern operating systems, it isn’t!
So my original comment kind of stands but in a opposite way.
I have never see file_9.txt sorted before file_10.txt, I just tested it on OpenBSD and I got this, which I have always seen:
$ ls|sort
file_1.txt
file_10.txt
file_12.txt
file_2.txt
file_20.txt
file_3.txt
file_9.txt