They also wanted control over the user base, so they could do more intrusive bullshit to push more ads onto users. With the fediverse there’s no monopoly on the platform so no one instance can get full control and abuse their power. With Reddit the only choice was to either submit or leave completely. With Lemmy all you need to do is swap instances.
Yeah that’s a great point. I think it would be hard to fully lock other clients out, but you could have an early internet style situation where you had some websites not supporting all browsers.