I think there is a much bigger problem and capital cost. Servers. 7 billion API requests per month from a third party app that's just one of them. There would be several. Then the much larger traffic from official sources. If you can't afford the servers moderation isn't even worth thinking about.
We're talking about a Reddit replacement. It's realistic to think a half decent one would get a 1 million or so users straight away within weeks. That's 7 billion requests.
BlueSky had to be invite only to stop getting too much traffic straight away. 1.5 million joined Mastodon when Twitter got bought. The numbers for a feasible replacement are still skyhigh at the start.
Hell you're assuming that an API even exists for people to build integrations against on day one, rather than the company just providing it's own UIs/APPs and scaling into APIs when it's ready.
It doesn't matter if it's web requests or API requests. Traffic is still traffic and it still needs to be handled by web servers that cost money. And the idea that a start up can go from 0 to 1 million users in a few weeks without a lot of cash for servers isn't realistic. No real competitor for Reddit would slowly grow, they either take advantage of people being unhappy and do so in a quick manner and have an intital surge or they don't even have a chance.
BlueSky has millions of users, people still complain that it's basically dead. Even though lots of people are using it. It's just dead compared to Twitter. That makes adoption harder.
Who is going to pay for all of those?