The pitch for federalization gives the misleading impression that the system provides a universal (or at least portable) account mechanism. It seems to be a common point of uncertainty with users taking a look at the emerging platform.
From a usability standpoint, the sign-in process ought to be able to tell when someone is trying to join from a non-local instance, and provide means to redirect or authenticate them appropriately. Maybe something in the style of "Sign in with Google," only simpler.
I think upcoming versions of Lemmy/Kbin are attempting to improve link behavior to make it less likely to unintentionally stray from your home instance.