Question: Trying to figure out the fallacy here, I can't put my thumb on it.

My interpretation of the statement:

Assertions without proof are absolutely True.

If I may cheat with an equivalent model:

Person A: I believe that God exists.
Person B: Why do you say so?
Person A: Because no one has ever proven that God does not exist.

I am reading LF for the first time, so I have no idea which one of the 300 violations this fits.