Imagine your mom emails you and asks why she can't access her favourite subreddit anymore and you have to explain she now has to make 2 different accounts to access different subreddits. I mean 'instances'. I just picture my own trying to wrap her head around "why".

It sounds like a minor hurdle to our demographic but most people are super lazy and bad at juggling online authentications. I would never recommend Mastodon to any non-tech person for that reason (beyond the other UI issues) unless there was a specific niche community they were interested in.

> can't I (a hypothetical human) just create a user account on each disconnected sub-graph

Managing multiple identities is challenging. It’s solved-problem challenging, but that implies a single client which sort of defeats the purpose of de-centralising the back end.

Yes, and what's more, you don't need to use a single account to manage an identity and all feeds like you would with a microblogging account, so it's not even a significant source of UX friction. These sorts of applications are tailor made for multiaccount usage, most reddit users even have more than one account to manage different communities.

Alternatively, one account on an instance that still federates with both halves of the split could still see content from both sides? One could even do this by running their own instance? Or is there something preventing this?

I think this is 'significant' due to a large volume of people who want Lemmy to be their reddit killer and replacer, regardless of the goals of the project itself.