I was only talking about the prize aspect of it. The rest is impossible to evaluate. All they say is that they'll make an offer of investment. That could either be good or worthless, depending on the amount, the valuation, and other terms.
FWIW, my guess is that they'd err on the side of ignoring good startups. Amazon has not so far been in the investment business, and I don't think that is their goal here. Their goal is to promote AWS to startups. They're raising the prospect of investment because they know that's something many startups are interested in.