I’d argue that Inferno describes a world that exist under our own, but it is still part of it, geographically and systemically, as all the souls of the damned eventually reach the underworld, while an Isekai story always (or most of the time, I should say) describe the other world as a parallel dimension, disconnected from our own and accessible only through supernatural or technological means, while Dante just have to walk through the gate.
I’d argue that Inferno describes a world that exist under our own, but it is still part of it, geographically and systemically, as all the souls of the damned eventually reach the underworld, while an Isekai story always (or most of the time, I should say) describe the other world as a parallel dimension, disconnected from our own and accessible only through supernatural or technological means, while Dante just have to walk through the gate.