First of all, that's not true. Not every position has to be empirically justified. I can reason about a position in all sorts of ways without testing. Here's an obvious example that requires no test at all:

1. Functional properties seem to arise from structural properties

2. Brains and LLMs have radically different structural properties

3. Two constructs with radically, fundamentally different structural properties are less likely to have identical functional properties

Therefor, my confidence in the belief that brains and LLMs should have identical functional properties is lowered by some amount, perhaps even just ever so slightly.

Not something I feel like fleshing out or defending, just an example of how I could reason about a position without testing it.

Second, I never said it wasn't testable.