Like I said, I’m inclined to agree with you, but I see a gap between the two things being contrasted. Is the OP only contrasting these two things (out of many), or are they it? That’s why I chose the example I did. Moldvay Basic was particularly convenient because it does have tables and procedures that can be used to discover as you play, but it’s not a game one’d expect to be put in the same bucket as PbtA or FitD games.I don't know, maybe @pemerton can say something about that. I think you cannot really judge by simply naming a system and positing some fairly broad style of play within it? This is all not a system-dependent thing, not exactly. I do still think that character exploration and engaging with the character-as-played, and allowing for genuine development of such in a way that feeds back into play is pretty much key. Its not going to be clear cut in all cases, by any means, IMHO.
I think there’s something more fundamental happening in the OP rather than “authentic” being just a euphemism for “narrativist”. If the characters set out to discover what’s out there, then they can’t also help but discover something about themselves as they face adversity. That seems to be exactly what @pemerton is saying in his second post explaining his use of authenticity.Going back and rereading the OP (this is something people should do more, IMHO) I see that there is a contrast drawn specifically between what I would generally consider primarily narrativist play and railroady "GM is presenting a story" kind of play. Again, there's not some perfectly clear demarcation there. As for "what is a sandbox?" I think its still not going to be a clean 'this' or 'that'. Obviously in your scenario above the characters can explore Mystara. That could be their major activity. What are the players doing though? Are they making choices that are significant to the characters? Or maybe more interesting, do they have choices that represent different possible paths of character development, and genuinely different outcomes in terms of who the PCs are and what becomes of them?
Authenticity: the property of being authentic. Authentic: issuing from and being true to the self; thus, revealing (something about, some aspect of) the self.