You don't need to trust the creators' claims - you're running their simulations, and you're damn good at understanding them and extrapolating the consequences because, well, you're superintelligent! Why would they even know they're simulated? They're just discussing one-shot PD on some blog.

As for the SI being a successful extrapolation, you run a few simulations of it's birth the same way, starting a few decades before. It's still cheaper and less messy than organizing a mutual reprogramming with the brain that's made of the next galaxy.

Then the problem largely reduces to:

• Verifying the data you passed each-other about your births are accurate.

• Verifying ethical treatment of each-others simulated creators - no "victory candescence" when you get your answer!