Maybe have a signup flow where you can skip the new account restriction by putting some file on a website of some currently trending link. And then the restriction is lifted temporarily for the thread linking to it?
Not every post is from the website of the person who is the topic of it. It's common to have e.g. a blogpost about $thing and then a new account chimes in with "Hey, I authored $thing 10 years ago when I was working for $company, someone linked me this post. [some contributions to the topic]"