Is this true (electronic first curtain) for other camera models as well (E.g. 5d3), or just the 1dx?
It's true for almost all of them, although with the higher end cameras (like the 7D, 5DII/III, 1-series) you have to have it set to the default Silent LV Shooting Mode 1, i.e. disabling Silent LV shooting means no electronic first curtain. With the lower end cameras (Rebel/xxxD, not sure about the xxD), EFC is always on for LV shooting.
It works if you just hold the button down after the shot and walk away :oP
Actually, the real
solution for a silent shooting mode on the 1D X is just to shoot from further away: