I think the highs are more typical of bipolar, though.
I don't know whether authors are more prone to depression than other people or not. And as for rejection slips, some kinds of depression can even buffer you. Mine dumps mountains of apathy on me much of the time. I don't enjoy anything, I don't want to do anything, I don't care what happens to me. On the other hand, if you have the kind where you are on the verge of hurting yourself, I don't doubt that rejection can make things worse.
But if people are depressed in other professions, I think it is easier to hide the symptoms. If you're in a management position you can pass all the work off on subordinates. If you have a mindless, repetitive job that requires no concentration or initiative, nobody notices because you can just plod on for a long time -- until and unless it gets so bad that you either withdraw from life entirely, or you do something desperate.
In writing, as in other arts, it can be more obvious. Usually, there is no one else to do your work for you if you can't concentrate, if the creative part of your brain has gone dull, if you can't even remember what day it is and what you planned to do. And you can't just plod along with your brain in a fog without a clear drop in the quality. Which only makes the lack of initiative worse if you know that nothing you do is any good. Although depression can also prevent you from realizing when it is good.
So perhaps it is more common with writers, or perhaps it's that the symptoms take a greater toll professionally than in other professions, so that people are more aware that something is wrong. It's hard to know, since even though there is more awareness now than there was in the past, depression is still widely misunderstood.