Aldarc
Legend
(1) I didn't write it. (2) This tells me that you should read the rules because it's getting tiring having to explain rules that are freely available on an SRD.From these words you wrote: "Remember, there's no 'to hit' in DW. A hard move like that will be serious damage or death."
The "will" in there tells me the move - and thus the attacker - cannot miss.
Cannot miss is way too strong and misunderstands what's going on. @AbdulAlhazred is saying that the reason that the attack will hit is because when you the GM use a hard move to inflict damage that you are fundamentally declaring that it hits through bypassing an important part of the resolution process: i.e., the entire player-side of the resolution process! This doesn't mean that the attacker is far too perfect. It means that you as a GM are not playing by the rules! (Which would make you a horrible referee!) Again, this is tantamount in D&D to you as a GM declaring that the sniper hits the PC without you first making an attack roll against the PC! For shame!
There is no to-hit by GMPCs, because the GM doesn't roll in PbtA games. Only players roll. Players react to the fiction that the GM frames. If the GM frames a soft move, for example, that goblins are shooting at the PC, the GM will ask the players what they do. The PC then tells the GM how they react. The player characters' actions may trigger a move roll, such as Defy Danger. In which case, the players may roll to avoid the damage in some way: e.g., take cover, raise a shield, dodge the arrows, etc. On a full success (10+), the PC likely will avoid all the damage. On a failure (6-), the GM can then make a hard move to Harm Them with damage. There is some flexibility in the results of a mixed success (7-9).