ActivityPub, the protocol that powers the fediverse (including Mastodon – same caveats as the first two times, will be used interchangeably, deal with it) is not private. It is not even semi-private. It is a completely public medium and absolutely nothing posted on it, including direct messages, can be seen as even remotely secure. Worse, anything you post on Mastodon is, once sent, for all intents and purposes completely irrevocable. To function, the network relies upon the good faith participation of thousands of independently owned and operated servers, but a bad actor simply has to behave not in good faith and there is absolutely no mechanism to stop them or to get around this. Worse, whatever legal protections are in place around personal data are either non-applicable or would be stunningly hard to enforce.
They hired 2000 staff because they were running one of the most highly-trafficked websites on the Internet, with hundreds of millions of users, something which unsurprisingly takes quite a lot of people to administer and maintain. Were Reddit to not invest in people and resources to keep the website running at that scale, you would not be able to use it in the way you enjoy and it would have nowhere near as much utility to you.
This is the entire point of the article that you missed - there are a shit-ton of costs in running a massive community that have to come from somewhere. Your approach is "well I don't think those costs are necessary". But they are.