At least when you submit short stories or the first three chapters of a novel you're looking at odds like 1 in a hundred, or 1 in sixty (if you're lucky), or even one in 240 for the major publications. But 1 in a thousand plus
Uh, the odds for getting published in a major publication or getting a novel published are far worse than 1 in a 1000.
Magazines, anthologies and publishing houses each receive thousands of submissions monthly and many only publish a handful of what they receive, and if you're an unpublished author you're odds of even getting your story read by an actual fiction editor are even less. Many of these magazines and publishers have people (assistant editiors) who are solely responsible for reading the first paragraph or two and deciding whether to toss it or not based on those first few words. If they get past the first couple of paragraphs and read the rest they still may toss it.
Your odds of making it as a writer are worse than the lottery.
Also, remember that this, like submitting a story or novel, isn't a contest. WotC has asked for submissions and are reviewing them so your odds increase\decrease based on how well you write, the quality of your ideas and how well you can follow instructions.
You might have the best idea for a setting in the world but if you did not follow instructions or you write poorly you will not get selected. Your odds are effectively 1 in a 1,000,000,000. The same goes for someone who is a great writer and followed instructions but has lousy ideas. They will not win.
So, the odds of winning this are not 1 in 10,000 for everyone. They are significantly better for some and worse for others.