narrow enough that it doesn't exclude a motivation to sim humans

Most UFAI will have convergent instrumental reasons to sim at least some humans, just as a component of simulating the universal in general towards better prediction/understanding.

FAI has that same small motivation plus the more direct end goal of creating enormous numbers of sims to satisfy human's highly convergent desire for an afterlife to exist. The creation of an immortal afterlife is the single most important defining characteristic of FAI. Humans have spent a huge amount of time thinking and debating about what kinds of gods should/could exist, and afterlife/immortality is the number one concern - and transhumanists are certainly no exception.