A lot of takes wrt walled gardens here and this vs threads, to which I could add my 2 cents. Facebook used to have its own things called Notes which was basically the same thing to this (longer format writing attached to your profile). In the years leading up to me launching my blog/personal website, I wrote about a half dozen of these Notes (this was like a decade ago, I was in my early 20s). It was a pretty good 'on ramp' to writing down longer sets of thoughts to share with people, much lower friction that launching a blog.
I doubt this is going to serve the same purpose for many people, but it might. Yes it is a walled garden, but I don't think it's meant as a replacement to a personal blog or Medium account (though tbh id much prefer this to Medium). I also have no doubt this will co-exist with Twitter threads, with the two having different strengths and weaknesses.
So, as someone who thinks Twitter is flawed but also very usable in a way that makes it quite good, I think it's a nice addition! We'll see if it survives - I kind of hope it does.