Is it true that depression is considered a neurological disorder?

Yes, but... All psychological/psychiatric disorders/symptoms are neurological in the sense that they are all expressed through changes in the brain and nervous system. In that sense depression is a neurological disorder. However, the difference between depression and say parkinson's disease or stroke is that in many instances of depression, talk therapy is effective.
Some depressions are. It is useful to think of depression as a symptom for this question. Causes of the symptom can vary from physiological (your interest) to situational with many between. Several neurological conditions can include this symptom - parkinson's, epilepsy, etc. Mri's reveal structural evidence in some w/ major depression - but i'd not expect it in mild or moderate depressions. Hope this helps.