Yes, it is not up to the church to pronounce on whether or not a person is a Christian.
If as you say, the excommunication had no effect, do you mean there is evidence that priests continued to give the sacraments to Nazi leaders after 1931 in defiance of the excommunication? I would find that surprising and I would like you to provide evidence if that is your contention. Or do you just mean it had no effect since
none of these people ever went to church in the first place? That seems to me to be highly likely - but obviously that is something completely outside the church's control.
By the way, a bit more on this from Quora:
Why didn't the Catholic Church excommunicate Hitler? - Quora. Seems the excommunication was actually appealed to Rome and the appeal was denied, according to one Professor of religious studies. Interesting - I had no idea about any of this.
But the whole thing is a bit academic since at least in Hitler's case he had left the church anyway, so he wasn't going to be wanting to take any sacraments. He had by that time excommunicated himself, in other words. So all this is a storm in a teacup, really.