What I like about this guestbook plugin is it has multi layer protection against spam, plus a final human review before anything goes live. Also, it does not try to use the Comments part of Wordpress. Somehow it just seems tidier to keep Comments, which in my mind are about posts, as comments and guestbook entries separate.
Heck it’s all part of an experiment: can you have old things like Guestbooks, will anybody know what they are, will they be useful, in 2018? The only way to tell is try it.
And it might lead to discovery of new websites for viewers.