Grey Oprelegon EX Mask ability can surpass pokemon who don’t take damage from EXs?

UntoldWendigo

I just fought somebody who had the grey Oprelegon EX that has an ability saying it doesn’t take damage from pokemon with rule boxes, and they used it against my Sylveon which doesn’t take damage from EXs. Previously, neither could damage one another, but this match it could!!! Is this a bug? Its ability says NOTHING about it cancelling out the ability of the other active pokemon.

UntoldWendigo

not rule box, I meant pokemon with abilities for the Oprelegon ability.