The proof is conditional upon an assumption. You are averring to the unproven nature of the assumption. Your detractors are averring to the proof which it conditions. Of course both are right, and of course both are wrong, in different senses.
Thanks!
Actually it is two independent assumptions: (1) solving the mining problem requires a lot more computation than checking the solution, and (2) forging the signature for a transaction requires an impractical amount of computation. AFAIK, either could be false with the other being true.