WHO stands for World Health Organization. I don't think anyone in their right senses can doubt their credibility. The 14-21 days figure sounds optimistic but scientifically speaking this much time period is really good enough for antibody detection using 3rd generation or higher antibody tests.
However, I am sure that you have seen on many websites that individual immune factor can be variable from 1 person to another. Some people develop antibodies faster and some later depending how healthy the immune system is. So, keeping all these things in mind, various health organizations just want to play it safe
by including Worst Case Scenarios when prescribing a window period. After all, they have to look at this all from a public health perspective rather than an individual health perspective. I hope this helps...