ActivityPub implementations generally don’t allow this.
This comment will, when I click ‘Reply’, be sent to your instance (dormi.zone), that instance should then run it’s filter/block checks on it and if it’s happy it will forward it onto the lemmy.ml instance for further disemination amongst the subscribers of the group.
If you were to have blocked me then my reply will appear on my instance only (which is admitedly tiny - at 1 user) and go no further. This kind of falls apart if I were to be on a bigger instance as more people would see the reply.
That said, Lemmy may not be doing that quite right as the whole Groups/Communities thing is sort of an extension of the main protocol. I hope it’s doing it the right way.
Ah, well that is indeed unfortunate and realistically also a bit shit.