The glow plugs in this engine are housed inside the combustion chamber right beneath the injector nozzles - and are no way in the inlet manifold.
Their purpose is to heat the combustion chamber aiding in heating the compressed air for diesel to get to ignition temperature as soon as it is sprayed.
What you are reffering to is usually found in industrial diesels which usually use no Glow plugs - but have heating elements in the inlet ram pipes to supply HOT air into the cylinders - same effect - different approach