But at some point it has been changed from being a Cities expansion to being a Leaders expansion, incorrectly in my opinion.
It's for sure expansion for Leaders. You can't play it without Leaders.
It's quite disputable if it's an expansion for Cities. It has a mask symbol (and Louis promo also has it) but it's not related in any way to mechanics and themes of Cities. It's just a stand alone leader with unique ability.
The better question would be, why there is a mask symbol on it (and on Louis). I think that most obvious answer is that they used the same template for Cities leaders and promo leaders. It just happened that they forgot to remove Cities expansion symbol from template.