Proofs rely on assumptions. In this case, they state outright:
> We first posit that the number of days T that a fact can be retained before it needs to be reviewed grows as a power-law in s, the number of times it’s been reviewed so far, ...
Obviously this assumption will be false in our physical universe, but that doesn't make the proof itself invalid (edited).
In logic, the soundness of a proof in fact has to do with its interpretation in some universe of discourse. To be sound, the argument has to be deductively valid, and its premises have to have true interpretations in the chosen world where it is applied.
Here we have a valid mathematical argument which is unsound in this world, where its assumptions do not hold up.
You're right. I should have said invalid rather than unsound. I still don't see the point of the criticism, however. Lots of interesting things can be learned by starting from approximations to actual reality.
> We first posit that the number of days T that a fact can be retained before it needs to be reviewed grows as a power-law in s, the number of times it’s been reviewed so far, ...
Obviously this assumption will be false in our physical universe, but that doesn't make the proof itself invalid (edited).