The point is you roll to hit against the unit and the majority weapon skill unless in a challenge
Therefore by the time Seth is involved (by having wounds allocated to him) it's already too late for his rule - you've done your hits and can't go back to start re-rolling
If he's in a challenge or he's a lone model being assaulted then fine, you have to hit him and only him so therefore his rule can come into play but if you're attacking a unit that contains him you're not attacking him directly, you're attacking his unit and the specific wording of the rule prohibits its use
Note also the wording of the FAQ question involving him:
"Q: Do models that are allowed to re-roll rolls of 1 To Hit in close
combat, who are fighting Gabriel Seth, suffer hits if they roll 1 To Hit
and further hits if their re-rolls To Hit are also 1s?"
If you're rolling to hit against the unit as per the rules in the Rulebook then you're not fighting Gabriel Seth, you're fighting a unit which happens to have Gabriel Seth in it, a different enough distinction to prevent the use of his special rule