I'm considering a facelift at 40. Is it true that a traditional facelift won't correct cheek sagging?

A facelift of any kind needs to be properly executed. Release of the retaining ligaments along the cheekbone as well as the jaw will permit excellent repositioning of the soft tissues of the upper face. A conventional full facelift also takes care of the neck all the way down to the clavicle.

That is correct.  A facelift does not treat the cheeks/midface but has most if its power in the jowls and neck.  Your options for your mid face would be a mid face lift, fillers or a cheek implant.  In my practice, I prefer cheek implants and/or fillers.  Please seek a personal consultation and exam with a board certified plastic surgeon.

There are many different options to provide cheek lift and enhancement. Most of cheek sagging is due to loss of cheek volume (fat) rather then true descend (sagging). In patient of your age volume enhancement with fat grafting, fillers or cheek augmentation with implants may be an option. Patient of Asian origin frequently have small, hypoplastic cheek bones and they present flat, concave cheeks sooner then those with normally developed cheek structures.You should seek a consultation with board certified plastic surgeon in your area for detailed evaluation and discussion. Good luck.