Accessing the content is not a problem, commenting as well. You can access it even if you’re not subscribed (in most cases, i.e. if the community is not private). The problem is the feed. You don’t get anything from that community in your feed unless you’re subscribed to it. Exceptions are communities that are local to your instance, you get feeds from them even if you’re not subscribed to them (since the default in Jerboa regarding feeds is Local feeds).