Are you not aware of Plato's Seventh Letter? There Plato is pretty darn clear, "No where is my philosophy written".
As to qualifying, I did actually by citing Heidegger and Gadamer, who give a DETAILED account of how the tradition took way too many liberties in assuming exactly what you assume. If you have not read them, then I am not sure you are in a position to challenge the claim. I did my Masters Thesis on Heidegger and my Dissertation discusses how William James embodies similar tendencies as we see in rereading Plato through the lenses of the anti-traditional view. I can give you links to them if you want to read them.
ETA: and I find it somewhat ironic that you have a quote from Nietzsche in your profile, since he and Jacob Burckhardt really started the movement to reread Plato in light of the possibility that Plato was NOT writing down HIS philosophy per se. In fact, as Plato argued, philosophy is not something that can be captured by writing in the first place. Paging Phaedrus . . .