The assesment that he’s the wealthiest person on earth is pretty dubious, actually. The analyses which list the worlds wealthiest people always are, because they have to decide what counts as wealth and how to count it.
Normally that’s fairly easy, but for very powerful people (who, as you point out, the people at the top of those lists are) it gets murky because of things like stocks and options which they could liquidate in theory, but which would crash in value if they tried to actually do so. Does it still count as wealth if it only exists so long as you don’t spend it?
There are also people who’s wealth isn’t held in any currency, or gold, or stocks. How do you measure the wealth or power of a sovereign king, or any other kind of dictator? You certainly can’t neatly put it in a scale alongside people who just have a dragon’s horde of cash somewhere, that wouldn’t be comparing like for like
That’s going to be a problem whatever solution you come up with, because of the federated nature of the lemmy system.
There’s no central authority to hand out usernames, so if two people sign up to different instances with the same username, any design which didn’t attach instance name to each username would fail. The only way around it would be for each instance to contact every other instance which exists, including the ones which haven’t federated yet, and negotiate ownership of the new username, and that’s just not possible