As the article states, the net impact depends entirely on the actual terms of the proposed sale--and on what Dynergy does with the funds it receives from the sale.
The situation is analogous to that of a landlord who owns a rent-controlled property. It may be that a prospective buyer would be interested in living on the property himself, and the law would permit the owner to evict the current tenant(s) in order to occupy the premises himself. The result would probably be to significantly increase the market value of the property, due to its being freed from its controlled rent.
Similarly, Dynergy is hoping to find a buyer for Illinois Power whose situation enables the buyer to operate the utility more profitably, and who would therefore be willing to buy it for a price that exceeds its worth to Dynergy.
I think (s)he came up with a very good analysis. Posted for later referral.