A sign-up wall for a site which should be able to show me public book reviews and recommendations is a non-starter for me (and probably 99% of users clicking the link). I really don't need another social media platform, I would like a book discovery system though.
I don't think reviews or recommendations are the goal of the site, as there is currently no way to add reviews or ratings, nor does the site provide any reviews or recommendations of its own. It's literally "just" a social network to talk about books on.
Most of the confusion would go away if they had read-only access for anonymous users. I suspect they chose to not do this because there just isn't much there yet.