That's absurd... Netherlands, Germany, and Italy have provided one of the best managers ever in history of Football, that's why most teams choose managers from those countries, not because they are white.
British managers are white too, and no proper top club wants to hire british managers nowadays, because most of them are outdated when it comes to tactics.
Also you seem to forget that there aren't many black managers out there, because not many black players choose the managerial career when they retire.
What's absurd about it? That's exactly the point, more managers are going to be chosen from those countries because of that and it's a self-fulfilling cycle. They have more chances to be successful than others, they have some success and then even more are chosen as a 'safe' pair of hands. If there is a similar level ability manager from Germany or from South Africa, which manager is chosen? It will be the German.
Why are there so few black managers, given how so many great players were black? There is a reason they don't choose that career, because things aren't put on a silver plate for them, they really have to graft too hard for opportunities. They simply don't have the 'right' look. A poor manager like Lampard gets lots of chances because he has the 'right' look and sounds right.
You're right it's not just a black thing, it's also Asian, Arab and other less heralded countries. To reference that Guardian article, Postecoglu was doubted when he first arrived at Celtic, despite being a good manager, because having managing in Japan and Australia was not enough.
Pitso Mosimane has won 5 leagues in South Africa, then moved to Asia and won 3 Asian Champions Leagues. Walid Regragui got Morocco to the semi-finals of the World Cup. But no sign of any opportunities for them.
“I think it’s impossible that Manchester City or Barcelona will bring an African or Arab coach, they don’t even think about it, as if we’re not worthy, as if we are ignorant in football, or we’re incapable of such a task,” he said.
Might come across as a little bitter or deluded for him to be getting those jobs - yet there is something in it about managers having to fit a certain profile. Which is why ex-players with no experience often end up ahead of the queue over managers that don't fit a certain profile.