The amount of miles you earn depends on how much you deposit into your Fidelity account.
$50,000 = 25k miles
$10,000 = 15k miles
$2,500 = 5k miles
The $2,500/5k miles deal seems to be the best option as you’ll be earning 2 miles for every dollar you deposit and you don’t have to commit $50,000.
More details at fidelity.com/united.
Yes, this is a great deal. I earned 25k United Airlines miles a couple of years ago, which paid for a free trip.
[If I were smart, I would have kept that money in cash, but that's another story.]