Interesting that in the title, stated in absolute terms in the text, and from the designers they interviewed, they cite getting lost as crucial for the genre. Personally, I disagree. Getting lost has tended to be why I didn’t care for certain games in this genre, like Axiom Verge, and it soured my otherwise higher opinion of games like Hollow Knight and Symphony of the Night. Still, I think this is a good exploration of the genre and what makes it tick.
I count Dark Souls as a Metroidvania in my head, honestly.
But I think the actual defining feature is unlocking new abilities to reach new locations.
DS1 has you unlocking new areas that are interconnected with ones previously explored, but you don’t really unlock new moves to get to a new place, it usually just happens after you beat a boss or buy an item.