Specifically, the $12.99/year model is floating a ton of free users, in addition to paying for server resource usage. None of that $12.99 currently was allocated to paying Reddit, and we know Apollo calculated an approximate cost of $2.50 per user per month with the new API fees.
This $2.50 fee seems to align costs with what we can reasonably expect Reddit to earn per user on their platform. Reddit prices Premium membership at $5.99 monthly, which among other benefits removes all ads. $5.99 likely indicates a $2-3 profit when all ads are removed but user engagement remains constant.
The typical mildly engaged Reddit users probably easily spends an hour a day looking at ads via the app - so while the math may be fuzzy, it seems like Reddit possibly based their API pricing off something like this.
At a minimum, that is $2.50 x 12 = $30 of cost annually per user. This means all users need to pay Apollo $30 a year to break even on just Reddit fees, or some subset of users needs to pay a lot more than $30 a year to float a bunch of free users. Apollo has other expenses too (labor, servers, etc.).
Even if the API fees were reduced 80% down to $0.50 per user per month, that's still $6 annually per user - and Apollo has a lot of free users.
All of this is to say, the $12.99/year membership for Apollo was never going to work with any API fees.