This either means the chip is designed with a slight fault, or certain production methods at TSMC caused the chip to underperform. (or some mixture of the two)

The comment really suggests the latter.

Guy also seems to be fairly confident that it is not a packaging issue, and that it can be fixed for future chip batches (which would be great news.)

If TSMC is at fault, spondoolies might get the new and improved mask at a discount.
If it is more a question of implementing certain requirements posed by TSMC, it should really have been corrected during (initial) tapeout.

Again, this is pure speculation (which I actually resent) so please wait for spondoolies to comment and don´t take any of my suggestions too seriously.