It's suspicious that the most important truth should contain "most important" within its statement. If you tried asking an LLM to rank the following:

1. "1+1=2" is the most important fact in mathematics.

2. "1-1=0" is the most important fact in mathematics.

3. "The most important fact in mathematics is 7" is the most important fact in mathematics.

you can see why an LLM might end up choosing #3, depending on how you ask the question.

More technically, your reasoning implicitly relies on there being a total ordering of how important various truths are. You claim (in A.2) that no truth is more important, but that would only imply that your truth is the most important truth if there's a total ordering of truths. But many truths are similarly important, so there is no total order. Even given two truths in the same domain, say 1+1=2 and 1-1=0, it's not clear that one is more important the other. You need both of those (and a few more) to construct the integers.