I'm sure the price is negotiable. Off the top of my head, the reasons you'd pay any nonzero amount of money for it are:

1. You avoid all of this drama. Christian makes a post that says, "Reddit offered me a lot of money and I said yes because I want to have a lot of money. It's been a pleasure working with them and it's been a pleasure developing Apollo for you. Peace out." Reddit ran this playbook with Alien Blue and it worked out.

2. Reddit could rather enshittify Apollo gradually, and/or fold Apollo's subscription model into Reddit's own model to maintain a premium power-user experience. It is an absurdly well-polished app, even if you added advertisements to it. It is a better user experience if the app gets worse slowly than if it disappears all at once and forces everyone to migrate to a new app. Reddit ran this playbook with Alien Blue and it worked out.

The goal of reddit here is to obviously shut down the app, he was saying "just pay me then" - he's obviously annoyed. Stupid thing for him to say for sure, it shut the "negotiation" down.