the closest of links is a poor choice of phrase here, for what is "the closest of links" but a direct link, so the writer should have used that simpler, more easily understood phrase if that's what he/she meant, or perhaps the writer could have stated that There is a close link between... if he/she wanted to be more general.
the closest of links is commonly used with the word not to express the opposite- that things are not closely linked.
A and B are directly linked in the English alphabet. D and L have not the closest of links.
Student or Learner