It seems to me the author doesn't really know what kind of story he wanted to make, he just have a pretty neat premise and then throwing whatever that sticks.
In a way he's an impostor because the real princess' personality technically died.
That's how it works with this kind of setup, like Honzuki no Gekokujou.
I love how MC is basically the badass old man trope.
This is the kind of hupe fight you can't really get out of LN adaptations due to the nature of the source material not allowing for much visual spectacle.
@vodkaisvodka He's missing half of his body, that motherfucker is dead.
I mean, he did transport to another world, did just beat the shit out of the fusion dance'd form of pedobear and Ghost Rider*, and the guy who was torn in half does have a giant mouth for an abdomen. You can give him some leeway for being skeptical about standard reality rules applying.