No, we don't know the antidepressive mechanism of antidepressants, therefore we don't know how they work.
Understanding some neurochemical reactions and biological adaptations they cause is not enough.
(We also don't have a complete understanding of depressive disorders, we don't really have very effective medication/therapy for many depressive people, etc.)
We know how antidepressants work, the chemical reaction is well known and can be quantified at different doses. We just don't know why they work because we don't understand why/how the brain works.
Pedantic but important difference.