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.