Suppose a "hidden truth" turned out not to have been intended by the author — would that make it less true?

One of the beautiful things about art (and life) is that it literally is what we make of it (or what we can make from it using the evidence at hand).

Authorial intent is super interesting, of course, and can enrich your understanding, but it's also pretty rare information. Often the work itself is all that remains after the author's gone.

At that point, it doesn't matter what the author "meant". You read what they left you to read and get whatever value from that you can.