Take a close look; previously, the Undead Realm could only store a large number of undead, but now it's different. The undead within can slowly enhance their power, which is already akin to a cultivation ability.
It's important to know that undead almost have no way to cultivate, or perhaps there are very few cultivation methods available.
Most undead lack the capacity for thought; even if they were provided with these cultivation methods, they wouldn't be able to learn them.
The ability to extend the existence time of summoned creatures, although seemingly unchanged, actually differs entirely. Previously, the undead creatures summoned could last for a day, but now they can last for several years.
Those that had a long duration previously can now last even more exaggeratedly.
