Not commenting on whether the 16MB information is correct or not, but grandparent's point is
not that only 16MB gets write-leveled; it's that for the purpose of write-leveling, the drive
is treated as a series of 16MB blocks, each of them are write-leveling within themselves.
(The write-leveling circuitry would then be much simpler -- imagine a parallel series of, say,
8-bit adders, compared to a 64-bit adders made up of 8-bit adders)