I agree, it's basically just a "hype/moral" boost for everyone around him and himself.
It's like a "valiant defeat", you still lost, but you made the best out of it.
It's commendable that the humans face a strong enemy at a disadvantage but that's about it.
(similar to how sports anime portray "hard workers" against "talented guys" that somehow manage to be relatively equal)
You could consider it a failure in the first place that they have to do that and can't force demons to fight during the day or make use of it, or force demons into it by smoking out their hideout.
It's not even as bad as a phyrric victory for the demons though, because they still killed one of the strongest fighters the humans have, while they themselve only lost minions.
I don't think the subtext was lost to Darth at all, it's just that when you are talking about winning and strength, the humans simply lost on all accounts other than "moral" and "duty"- they are just the "winner of hearts"
But Tanjiro's taunts mean very little in the grim face of reality and can be translated to 2 guys playing a video game and one of them saying "You only won against me because you used OP units/weapons and we almost won against you anyway!"
These weapons were always part of the game though and everyone knew they exist, even if they are overpowered.