ehh ... everything we measure relies on our understanding of the universe in some way. It's perfectly reasonable that our distance measurements could rely on a shakier foundation of assumptions than our understanding of the CMB. I don't know enough to say one way or the other, but GP's comment is not unreasonable on its face. Whereas talking about "provable" in cosmology, and certainly in this case, does seem unreasonable -- especially with error bars, which by definition have a small chance of not including the real value. Normally I'd say that we can generally refine our assumptions to be extremely good, and just take more measurements, and keep narrowing that error bar, until we hit a level of certainty that anyone reasonable would call "proven", but the entire point of TFA is that this isn't happening in this case. We seem on our way to "proving" two inconsistent things.