Adashino-san wa Sude ni Shinderu. - Vol. 1 Ch. 7 - Headless? What's that? Sounds scary

[MangaDex]

 

[MacMeaties]

Fairies; dullahan are unsellie court (malevolent) fairies, not undead.

 

[Pachka84]

"This is my real power!" Hahahaha.

 

[Glomoro]

Fairies; dullahan are unsellie court (malevolent) fairies, not undead.

This story already went over the fact that the zombies in this world are not the same as zombies from the origin legend it mentioned, or even the movies the character likes. You could infer from this that perhaps, just maybe, this story also intends to say the same of other "undead."

But also Japan just generally considers dullahan to be undead to begin with for their purposes. They're super common in video games, manga, and anime as a type of undead. So the whole country was already pretty much rocking a "Our Dullahan are Different" trope most of the time. The other times, they're pretty much some generic catch all terms like monster girl or demi-human.