I think people are just reading things into the leaders that aren't there. Other than their fixed agenda and whatever intrinsic bonuses come from their civ uniques, they behave interchangeably with no definable personality. In my current game Brazil is mostly peaceful, they're the science leader and are aiming at a science victory, while having next to no culture output (other than me, the main peaceful culture-focused leader in this game is Gorgo). In other games they've been culture focused, strong or weak, and ranging from passive to highly aggressive. Every other civ behaves in exactly the same way. Slight personality differences do apparently exist, but they're much less evident than in past Civ games: there will be a slight tendency for Brazil to be powerful and culture-focused more often than not, but that's the extent of it.