This isn't true at all. I just double checked and you can view the thread logged out no problem. Twitter's UI is admittedly garbo so depending when you clicked on the thread you may have needed to click an expand link.
It's not consistent, I've been getting these login-walls for a while on Twitter when browsing incognito and now it started to show up on my normal windows if I'm logged out.
It's some kind of A/B test or progressive rollout, they are definitely forcing me to login to view threads and there is no way around it on the official Twitter website. I've been using nitter.net instead.
