"We've guessed wrong historically on data sizes, and if we had 9 bit bytes those guesses (if otherwise unchanged) would have been less wrong, so 9 bit bytes would be better!" is an extremely tenuous argument. Different decisions would have been made.

We need to be better at estimating require sizes, not trying to trick ourselves into accomplishing that by slipping in an extra bit to our bytes.

Author here. The argument was that by numerological coincidence, a couple of very important numbers (world population, written characters, seconds in an epoch, and plausible process memory usage) just happen to lie right near 2^16 / 2^32. I couldn't think of equally important numbers (for a computer) near ~260k or ~64B. We just got unlucky with the choice of 8-bit bytes.