Depends, really... The problem with Commorragh is its size and population. While the Dark city itself may be large as an entire planet, there's probably hundreds of milions of DE in it, if not billions. Not counting xeno mercenaries and so on.
DE can also close off and isolate portions of Commorragh... so it's questionable if a legion would be able to effectively hunt them all down and kill....
Ofc, that doesn't means a legion wouldn't stand a chance, it's afterall at least 100,000 marines we are talking about.
Even if Imperium managed to defeat most of Commorragh, I guess the remaining DE would flee. Eldar aren't really known for static wars and last stands.