> is physics highly constrained by what can be observed in the universe

It is also very highly constrained by how _we_ observe the universe. Beings with different sensory/cognitive capacities could develop very different models.

> equivalences between apparently dissimilar modes of presentation

If there was some mathematical equivalence between their models and ours, which is already a leap to assume, there is still a question about whether the specific measure used would be translated to something equivalent to our object length measure in their model, which gets much stronger than just some equivalence assumption. And it’s even stronger to assume that this equivalence could just be inferred without any other information apart from the disk.

It seems disproportionate to fuss about "a leap to assume..." when we are talking about a small plaque affixed to a probe on the highly speculative basis that something intelligent might one day retrieve it, as opposed to something that is mission-critical. Would we be better off for not making these "leaps"?