Looking through that link I saw this:
Something else to think about: submitting to a contest takes your work off the market, at least temporarily, since most contests demand exclusive submission. It always amazes me that writers who grouse about agents who want three-month exclusives will happily consign their manuscripts to contest oblivion for much longer periods of time.
... which leads me back to the new thread I just started in the FAQ forum about understanding multiple submissions.
Although it doesn't seem to apply to this Hemingway contest, since they say:
We do accept simultaneous submissions; however, the writer must notify us if a story is accepted for publication or wins an award prior to our July announcements.
Quite a few contests (in fact, seems to be the majority from my own research) will accept simultaneous submissions, with the same caveat. And yes, you do have to sign over some rights temporarily. Most of the mentions of it I've seen are for 12 months, but it varies. But it's not as though your story is hidden away - it is out there for the public to see. And I think that's one of the good things about contests - it gets your name out there. Get a few well thought of contests under your belt, and it certainly adds interest when you go to an agent or publisher, even if you don't win first prize. Not to mention that many of the literary magazines that sponsor these contests also accept non-contest submissions.