Additionally, someone in an external instance can pull older posts by searching their links in the search page, often even if you get the link from yet another instance. Same for comments.

Only caveat is if the original post is unreachable, e.g. if communications were blocked ("instance defederated"), DNS or firewall is misbehaving at instance level, the instance is dead, etc.

Also, boosting, a microblogging thing, allows spreading posts and comments further, though a bit more indirectly.