No…I think I said they do advance, or can advance..typically they just take a really long time to do it but they have time. And a great number of them never do, just like many humans never rise to become CEO’s, Senators, Presidents, Sultans, Cardinals and Popes..
I wouldn’t go so far as to say there aren’t some demons who do exactly what you suggest i terms of sould consumption. However, most would have no real idea of how to do so. Remember animus and mana are not physical, so it’s not going to be clear “how” one would consume or assimilate other’s animus or mana. There are a number of creatures, particularly undead that do this naturally as part of their design…but it’s not something demons come built in with. If they could figure out how to do it, or someone taught them, they probably would do it.
But remember, assuming some super high powered demons have figured out how to do this, legends say that they do know how and do do this, they aren’t going to share this information with others because it’s a race for power. You don’t want to give the other demons any advantages.