It is a disservice to Plato to assume he lacks the ability to make a distinction between a (written) correspondence and text.
Plato's statement here is likely more subtle than superficially apparent. Specifically, note that he refers comparatively to painting, which (certainly in his day) required a subject (whether mental or natural).
"[W]riting shares a strange feature with painting", in that dimensions are collapsed; the subject is 'framed'; and viewer perspective (e.g. "if anyone asks them anything") is rendered irrelevant and "continues to signify just that very same thing forever".
Plato's statement here is likely more subtle than superficially apparent. Specifically, note that he refers comparatively to painting, which (certainly in his day) required a subject (whether mental or natural).
"[W]riting shares a strange feature with painting", in that dimensions are collapsed; the subject is 'framed'; and viewer perspective (e.g. "if anyone asks them anything") is rendered irrelevant and "continues to signify just that very same thing forever".