Yes it’s invite only. Yes that is a strategy to draw in users, potentially more effectively than being wide open. It’s in the article.
It doesn't matter how attractive a platform is if you simply aren't going to get onto it. Clubhouse can make me interested, but can't actually get me onboarded.
I can imagine a scenario though, were they gradually increase the number of invites they give out. I think the idea is that the perception of scarcity will persist, even when invites aren't really scarce.