It does not say that the FaceID will work if you sometimes wear glasses and sometimes wear contacts. The issue is not that glasses block light, but that they bend light, thereby potentially distorting the apparent 3D shape of the face. Perhaps FaceID will work no matter how you correct your vision if there is enough information in the lower part of the face and can learn to discount the upper half of the face. Perhaps it will generate two representations of the face (one with glasses, one without) in the hidden layers of the neural network processing the faces, but if so, this is information Apple has not provided so far as I am aware. Finally, infra-red will bounce off the frames of glasses, so there might be an issue if you wear more than one pair (normal glasses indoors, sunglasses outdoors) or if your glasses simply slip down your nose (moving relative to other features of the face). I guess we'll see soon enough once the phone is released.
