The robots.txt looks fine to me; IA’s crawler should be able to discover and archive any topic pages it likes. It looks like IA’s crawler just hasn’t decided to archive very many topic pages, for whatever reason, but there are some. Here’s an example.
If someone representing the website could email email@example.com, maybe they could adjust some configuration to make their crawler more likely to archive all the topics here.
Edit: I tried requesting that IA archive a topic page through their web UI, and IA did archive it (link), but the server didn’t give it the actual content of the topic; instead it returned “Oops! That page doesn’t exist or is private.” Might be a bug in Discourse? Or it could be some intentional bot blocking code within Discourse, possibly with a rate limit that IA’s crawler sometimes exceeds.