The tail thing and the wing thing are not true unless those qualities are specifically bred for, which they aren't in almost every breed or hybrid. Amberlinks are specifically bred to be feather sexed, so the wing thing does work with them.
I've never vent sexed chicks, so I cannot comment on that aspect. But I am suspicious. They say vent sexing needs to be done shortly after birth, and those chicks are older. Remember, male chicks do not have an appendage like male mammals do, so I think it would be very tricky to tell unless you were an expert at vent sexing.
