You aren't wrong.. those black chicks with white on head ARE cockerels... seems you had a strongly male run! Try to do things the opposite way next time for all female hatch eh?

(kidding! I once hatched 13 males and one female in one batch!)
The usual info on sex linking tends to be very simple as not to be confusing for a beginner. So they stick to breeds every one is familiar with for simplicity's sake.
It's directly the barring gene that is sex linked. Not the breed.
Barring expresses best on a solid black chicken, both the chick down and adult patterning-a barred rock is actually a solid black chicken with the barring gene added. The basic black chicken color is a dominant(and not sex linked), this is why hatcheries use barred rock hens for black sex links- the dominant black from the BR hen would make all chicks "black downed" with the males getting the white head spot that would show up well for easy visual sexing.
Barring generally not as strongly present on various colors and patternings with some exceptions. For example on a blue chick, the head spot can seem to be absent.. it gets 'diluted' sometimes as the blue gene likes to dilute pigments so the contrast can get lost. The head spot often is not very visible on some chick downs like some wheaten(cream) etc due to lack of contrast. In those cases you just have to wait until the chicks feather out to see the sex linkage.
For example if you used a cream leg bar with a black rooster, the chicks would come black downed with the males having the head spot plain for all to see. But if you used a RIR rooster over the legbar hen, you might be able to see head spots on some.. on others it would be so vague or even seemingly absent. However, as they feathered out, the barring on males would be evident. Not very economical for hatcheries to use.