Knockdown immunity 'does' apply to 'sending zombies 'flying'' with shock mods but it does NOT apply to other elemental or CC effects, such as KO, 'funky chicken dance' when on fire or electroshocked etc.
As you can see in Dr.Kesslers files, it'll list which zombies are immune to knock downs so you can use that for reference. Well it was fun in DI1 sending thugs flying into orbit, then stomping on their heads when they land.