There is inequality in the world and it is as it has always been, that the rich and more affluent rise above the common person and thus a gap is created between the rich and the poor. What Wilkinson is speaking of is not necessarily just who is rich and who is poor but rather the drastic effects that can take place in a society where economic inequality is allowed to continue. If there is no measure of equality between a societies class structures then it is clearly seen that problems both economic and social in nature begin to arise.
Crime, life expectancy, and even infant mortality are all highly affected by economic inequality as those who can afford a better class of life are typically those will live longer and stay healthier. The correlation between countries and who is more equal than others is not absolute, but in accordance with the data shown it is highly probable that countries with higher inequalities between social classes struggle more with serious concerns that eventually begin to affect every social class from the lowest to the highest. And yet, somehow, several countries have deduced how to maintain that equality and thus improve or maintain their way of life.
In regards to developing countries the inequalities felt are likely to be even worse, as those wealthier countries that Wilkinson highlights have all gone through their growing pains and despite their current inequalities have found a system that is proven enough to work. Likely as not within developing countries those who are without means to care for themselves will remain that way, and become even more downtrodden as those few who gain affluence do whatever they can to keep it. The relevance of what Wilkinson has to say is small when concerning still-developing countries, as the correlation between economic inequality and nationwide concerns is very small, almost to the point of non-existence given that such countries are still struggling to find an economic identity.