Loki and Kaido are no more of a dragon than Kings flames lmfao. They are men who can take the appearance/properties of a dragon, and some of it´s abilities, but they´re literally not dragons, like how a logia or paramecia user does not literally become their element....
The editor comment is meaningless, it´s just a description of what happened at the end of the chapter and trying to make it sound cool. Yes, Zoro could literally kill dragons since PH, but they added the comment because he cut a ¨dragon¨ in that chapter because it sounds cool, they aren´t thinking deeply or significantly about the story at all, and the fact that they say something like ¨sanjis legs are burning hot¨ is just describing what happened in the chapter, and there´s nothing more interesting happening(as if Sanji couldn´t have burning hot legs since prior to that in the story anyway).
This is just an assumption, Loki was tied up to seastone and couldn´t defend himself, nor use his devil fruit at all. There´s literally no reason to believe any character in the series wouldn´t be injured in that situation, with the likes of Luffy nearly passing out from Seastone touching him at some point in the series.