Determining gender by feathers is a trait that must be bred for. It is not applicable to all, and honestly to most chicken breeds. This genetic trait was bred into high production commercial lines so that big business farms could quickly discard males and not waste money feeding them. Orpingtons are not one of these commercial breeds.
I saw that video. A while back I also saw the write-up it was originally attached to but someone separated the write-up and the video. Now when people see that video they have no idea that the genetics have to be set up properly as Pele said.
If you are interested you can read the first post in this thread. Tim goes into really good detail aboput how the parents have to be set up genetically for feather-sexing to work.
There are two types of feather sexing. There is a sex-link method where a fast feathering and a slow feathering chicken are crossed which makes the first generation show a sex-linked feathering speed difference so you will hear a lot of people saying that you can't feather sex pure bred chickens. On the other hand, some breeds have pullets which feather faster than the cockerels...so with those breeds you can feather sex within the first few days and maybe even later on. Now, this trait might even vary within strains of breeds and I haven't done a lot of testing of this across breeds and strains but for me, with my Buff Orpingtons I can accurately sex them within the first few days as the pullets will feather out faster. There have been times that I was certain that it must not work as the numbers were skewed crazily in one direction or the other only to find out that sometimes I hatch out tons more pullets and sometimes tons more cockerels (it might have to do with incubator temperature killing off one sex embryo or the other). How else can I explain the lady who bought 37 buff orpington chicks "strain run" only to end up with 31 pullets and only 6 cockerels! I haven't noticed any difference with the other breeds that I raise.