They shrink from the “morning” light, as much as other men do from the blackest darkness (“the shadow of death”).
if one know - that is, recognize them. Rather, “They know well (are familiar with) the terrors of,” etc. [Umbreit]. Or, as Maurer, “They know the terrors of (this) darkness,” namely, of morning, the light, which is as terrible to them as darkness (“the shadow of death”) is to other men.