Maybe I'm misunderstanding how it's designed but I don't think I am, and I don't think that's how this works.

A topic definition on the wiki includes the instance it's hosted on. All links to that topic will go to that same instance and all the content for that topic will be served by the one instance as the authoritative source for "That-topic@that-instance" which is the link everyone will use. The federated part is specifically that you can link to topics on other instances and view them through your local instance.

For example, hypothetically, if you are a "fedipedia" author and you are writing a "fedipedia" article about a video game, and you mention a particular feature of the video game, you can include in your "fedipedia" article a link to a topic about that particular feature on "wikia-gamipedia" or even "the-games-own-wiki.site" and interact with and maybe even edit that content without needing to make accounts on all these other wikis. It's like it's all hosted on one centralized wiki, but it's hosted on different servers that are all talking to each other.

Of course, it's possible both our hypothetical "wikia-gamipedia" AND "the-games-own-wiki.site" will have their OWN, completely SEPARATE topics about the video game feature in question. The topics might even have exactly the same name. That's allowed. In that case, you'll have to decide for yourself which one is more credible and useful, and which one you want to link to and interact with, because yes, two different federated wikis can have different topics with totally different content.

Just like on Lemmy you can have two different communities with the same name but totally different people and content because they're on different instances. That's not really the general intention of how communities are supposed to work though. The intention is that you can pick the one community that is the "right" one for you, or the largest, and use that and hopefully other people will do the same. You can all pick that same instance/community, no matter which account you live on, even if it's not hosted on your local instance. You don't have to use the one from your local instance, or from any particular instance. That's what the federation does.