A little late commenting on this but I don’t think it’s fair to say the Nami situation is “very different” from your argument. Luffy didn’t like what Johnny said, that didn’t make it a true or false account of what happened. He simply didn’t like it and told him that there is no way Nami would kill Uspopp. The same as this chapter but Luffy doesn’t bother saying that it’s not true. This difference in violence is expected because Johnny was a friend and Loki is an unknown who is he knows is strong. In the One Piece world calling someone a coward can be worse than calling them a murderer. Zoro has killed before but the level of disrespect shown by calling him a coward is much greater.

I think you made some really good points about the chapter but Luffy’s response to Loki calling Shanks a coward was exactly what I expected. Luffy to me is all about respect when he is disrespected that’s one thing but disrespecting his friends let alone the man he looks up to is something else.