I don't think anyone should be surprised at the aggressive nature of stalsbergi, or any of the cichlids from west of the Andes, compared to Amazonian types east of the Andes.
In the east, there are hundreds of cichlid species, evolved to live together in varied crowded communities.
West of the Andes, these are less than a dozen cichlid species, usually living far from each other..
When you compare other similar loner cichlids from around the world, aggression is also similar, such as M beanii from northern Mexico, or N haitienesus the lone cichlid from the island of Haiti, both intolerant of other cichlids in even large tanks, both the only cichlids in their respective habitats.
And then consider the rift lake Africans that live in various crowded conglomerations, where crowded tanks are almost mandatory.