I don't know about random level generation or opponent spawning, but you should certainly be able to create quests and levels. There aren't any details yet on how fundamental you can get with mods, but it seems the intent is to allow fairly elaborate construction.

From the RPGWatch article (from immediately before the part you quoted)
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The same toolset as is used by the developers will be delivered as part of the game. Also, all the objects, tile sets and characters in the game will be delivered together with the toolset.

In the video on the D:OS site, Swen said
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Players will be able to make their own adventures, they will be able to share them online, they will be able to play with their friends and I hope that there's going to be a large community of new stories arising from that.

In the blog post Divinity � Original Sin revealed ! and one of its comments
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Personally, I�m hoping that the editor that ships with the game will be good enough to seed a productive modding community, so that I can finally play one of my own games without knowing what�s going to happen & this together with my girlfriend.
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We don't have any DLC plans except for allowing people who make really good stories that offer hours of play time to monetize their efforts


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