This is inaccurate(to be polite)."at 3 to 8 weeks, depending upon the breed, peck-order fighting will get serious and sexual activity will start. If you haven't already done so, it's time to separate the cockerels from the pullets or at least to pare down the number of cockerels to a reasonable ratio for the number of pullets."
For most breeds, males will 'show' by about 6 weeks by comb and wattle size and color.
They won't be sexually active until about 4 months, but that can vary.
Pecking order can start at one day...but can be fluid and changing forever.
Everything you read in books, and on forums, is not carved in stone.
Read a lot and bring your grains of salt.....lots of different opinions, techniques, and beliefs. Some work some do not.