While I very much hope AC is all good and has been sandbagging with big plans to win the Vuelta, I think the far more likely scenario is that he meant what he said - show up and honor the race, but only target stage wins.
Sure, AC usually rides only to try to win. But he's coming off a broken leg. In my opinion, he's riding this to get more race KMs so he doesn't go into next season having not raced a single GT this year (given how early he crashed out of the Tour).
Hopefully he can get a stage or two, pick up a few more points for the team, trounce Froome a little bit here or there to gain in the mental battles, and set himself in good stead physically for next year. Hopefully Oleg and the team are not rushing him and he's really ready.
And if I am wrong, and he was sandbagging all along and really tries to win the whole thing, I'll be very, very happy to have been wrong.