But, they are, right? They aren't preventing ALL ad blockers, but they aren't even merely preventing theoretical ad blockers: they are blocking concrete ad blockers that people have built and would actively use.
If there is a checkpoint at an event which seems to be turning away cars it doesn't matter if other cars are also getting through: the way we use the relevant phrasing allows us to say that "they are turning away cars" as that is a bit ambiguous as to why or how many.
But you do not need to discuss that. Rather suppose ad-blockers are working less effectively under v3, leading to additional ads being shown. That would increase the attack surface.
If ad-blockers really will be less effective under v3 remains to be seen. But do you think it is an unreasonable perspective, given the technical limitation?
Given that the reasons behind Google breaking backward compatibility are obviously primarily because they want to weaken ad blocking, I think it's perfectly reasonable to talk about this.
If there is a checkpoint at an event which seems to be turning away cars it doesn't matter if other cars are also getting through: the way we use the relevant phrasing allows us to say that "they are turning away cars" as that is a bit ambiguous as to why or how many.