There's no such thing as "sealing in the juices." Searing is only meant to add flavor via maillard reaction aka browning/caramelizing. The level of moisture loss is almost completely determinant on its internal temperature. That's why you see recipes in which a roast is started in a low oven (say 275 degrees) and finished in a hot pan or a hot oven (say 500 degrees) and still comes out juicy without having to "seal" it first.
Where "velveting" comes into play, is as a protective coating. It acts as a buffer for heat to prevent the surface of the chicken from overcooking and toughening. Doesn't actually "tenderize" the chicken, just making it seem more tender as it has not been overcooked.