Dave Wardale's response: Not in the thermodynamic sense that preheating is understood, i.e. as reclamation of otherwise waste heat, as the heating effect in this case comes from the surrounding boiler water/steam before this has done any work in the cylinders. It is therefore merely a part of the boiler evaporative heat transfer. The reason for such an arrangement was probably to precipitate scale on the feed-pipe walls, the pipe presumably being fairly easy to remove and descale (or descale in-situ by removing the clack-valve).
Note: this type of feed delivery would probably not be ideal with Porta-type feed-water treatment.