Quoted from Kieran in the "update post" here ( And I think that was quoted from TW.com):

Additionally, as with the recent note on naval invasions. The basic fix is in, meaning the AI can now choose this tactic and may do so. However, it will likely be rare until we've played with all the associated values (There are a lot). We started work on the next upgrade patch in parallel a few weeks ago and I can say for certain this area will continue to improve, so please stay with us.
So yeah, it's there but not finished yet, they are still working on it!