I'm not much of a Death of the Author sort of person, myself. Especially in the age of the Internet, the author can sometimes get into a situation where no matter how much they want to be dead with respect to their work, they are consistently revived in it and unable to escape being present while it is at the table instead of merely when it is being made. Commentary tracks, development blogs, and help threads all put massive holes in the idea that the author's intent can vanish into the ether no matter how strongly we preference play at the table from an analytical perspective.
I don't think there's anything wrong with having a platonic session. I suppose I got stuck on the implication that many games seemed to be missing a way of conveying that to the players or at least that, from the perspective that they have a platonic session somewhere in abstract space, they lack the tools to lure players sufficiently close to it. It's that aspect--the Platonic Session as a critical tool--that has me a bit lost. I certainly see why it can be a useful way to think about how to develop a good tool-set for players. I'm just also tempted to point out that shorter, more focused story games doing a more consistent and focused job of this is a feature of those games more so than a bug in other games.
I don't have anything more to add here other than that I don't really consider the Platonic Session to be a critical tool - I think I'm largely the only designer to actively use this lens and plenty of great games are being made without a cohesive and singular vision. It's just something vital to my own personal work process and something I think many designers would benefit from is all.
Otherwise I think we might be at a bit of an endpoint here. Thank you for the discussion! It's forced me to rethink a lot of the specifics behind the idea and the way I present it, I really appreciate your viewpoint.