In our last game a penitent engine assaulted through terrain a unit of Necron Praetorians. My Opponent claimed that the Engine having no access to grenades it should fight at initiative 1 and supported his claim through the
BRB chapters of Walker Movement and Assault chapters.
Now my question is why I haven't this elsewere in the past 3 editions of
WH40k. Has there been a change in the rules that i missed? Am I remembering older editions again

Or are walkers in melee subject to another rule that allows them to fight at initiative.
Thanks in advance for your answers...