If feddit.uk doesn't know about rabbitea.rs, then it won't send it there. The originating instance is responsible for sending it out, but it can only send to instances it knows are subscribed. This can result in some instances not getting new content if it originates on an instance other than the one that is the "home" for a magazine.
And surely the destination magazine must be involved somehow to run rules on like, banned users?
That depends. It depends on if the "home" instance federates out blocks. I don't actually know if kbin federates out blocks (many Fediverse platforms don't actually because it's used as a vector for harassment and dogpiling and other very harmful behaviors), but the 2 cases are as follows:
If Kbin (or the "home" instance platform) federates out blocks, then all subscribing instances will receive the block information. Whether or not they honor it or not is not guaranteed, but for simplicity sake, let's assume they do.
In this scenario, instances that receive the block will behave as you might expect: they won't allow the blocked user to view the content in the magazine/community but will allow other users to view it. They also will not accept content into their local copy of the magazine from the blocked user.
If kbin (or the "home" instance platform) does not federate out blocks, then other instances cannot know not to display content from a blocked account, nor do they know not to accept content created by a blocked account. The "home" instance will block such content from being displayed and added to the magazine and thus users on the "home" instance will not see the content, but users on other instances could, in theory, still see the content.
Note: This is partially why the mod_log can be viewed. I'm not 100% sure of the details, but the home instance moderation decisions should be visible on other instances (or at least by visiting the magazine on the "home" instance itself) and moderators/admins should be able to take similar actions on their own local-copy of the magazine.