Is there a word to describe a claim that cannot be disproved because the situation will never occur?
Such a debate is academic:
.7. Theoretical or speculative without a practical purpose or intention. [AHD]
.3. theoretical or hypothetical; not practical or directly useful: an academic question. [RHK Webster's]
The unrealistic aspect is perhaps best described by fanciful.
You could consider the claim moot.
Having little or no practical relevance
As you say, practically speaking, their belief in the claim is likely never going to be challenged, so their contention that they wouldn't eat an animal even at the cost of their own life is a moot one.