Your numbers are wildly outthere.
$2.50 would be the API costs per user. So $5 would cover, take in some money and pay appstore feeds. And if you're looking at 10% of his userbase, that would be about 4 million a month income with 2 million being for Reddit. Based off his claims of 20 million a month with a $2.50 cost per user.
I'm sure he'll be able to survive off $1,000,000 a month profit.
Edit corrected the numbers:
$2.50 a user for API costs.
$5.00 a month subscription would cover that cost.
$1.50 going to AppleTax.
$1.00 a user to the indie hacker
If 5% of users that would cost $20MM sign up for $5 a month it would be $2,000,000 a month overall revenue.
$1,000,000 to Reddit.
$600,000 to AppleTax
$400,000 a month to the indie hacker.