The following isn't blowing my own trumpet, but is framing for my perspective. The last IQ test I had (for what it's worth, I put little stead in their worth, and was made to do it by a concerned employer) placed me in the "profoundly gifted" category - just north of 180.
I have been outright miserable for my entire existence. As I've learned to better understand my own behaviour and impulses in the context of how humans generally work, it has become increasingly evident that my dissatisfaction stems from an excess of analytical thought, particularly when applied to "what if" scenarios.
This leaves me usually exceptionally well prepared for most situations life throws at me, through my incessant internal roleplaying, however also results in me "experiencing" many negative situations and outcomes, in order to understand how to either mitigate against them or to deal with them when they arise - and the emotions that come with them.
This results in a fixation on negative outcomes, which in turn feeds into anxiety, paranoia, and depression - as negative outcomes are far more common in most situations than positive - and mandate more attention so that they can be avoided.
In any case - I'd argue that creativity doesn't stem from mental illness, and that in fact depression as reported in many of these individuals (particularly given the absence of schizoid traits) stems from a similar source, in overanalysing situations. It's a correlating factor, not a causative one.
There's also the question of association, which is noted in the article, which I can to some extent vouch for. Much of my utility in what I do for my day job arises from being able to make associations from the aether that others don't seem to see, and posit solutions to multi-variable problems rapidly. It's a disconcerting feeling when you delve - something akin to turning off a chunk of rational thought, and letting the answers emerge from somewhere in my visual memory/thought processes. Also, outside of general problem solving, this results in strange associations, which usually emerge from me as baroque and obtuse metaphors - which again feed to the "mental illness" angle, it's somewhat inevitable that "this dude is nuts" is the conclusion others would and do take.
Anyway, just my $0.02 autopsychoanalysis.
Oh, and dysthymia was what they decided to diagnose me with when I last saw a shrink. Whatever.
Edit: just occurred to me as relevant to this - I had my genome done last year, and I have completely buggered serotonin and dopamine systems, and an overamped oxycontin system. Whether this feeds into this, I don't know, but I would suspect so. I get no sense of achievement from anything, which pushes me to always do more, learn more, be more.