I take it to mean they want the system to know file_9.txt is less then file_10.txt.
I never saw that happen in any OS, so I do not know what he is referring to. Maybe whatever that old system was, it sorted by create time as opposed to file name.
So, the author can try and create "aisort" that will look at all file names and add leading zeros to the file numeric portion, sort, then remove the zeros added. That will probably as slow as s***t and use gobs pf memory, depending on the number of files.