Feather sexing only works when it is specifically bred for. Amberlinks are one hybrid bred for this trait, but the vast majority of chickens cannot be accurately gendered this way.
The best way is to watch the combs and wattles for size and color. Usually 6 weeks is a good starting point.
Yeah, feather sexing, or slow vs fast feathering, is no better than random guessing unless you know for a fact that the breeder is selecting for it. And for most breeds, a hatchery is not breeding for that.