It's a typo, they meant ~2²¹ instead of 2²¹, because it's 17*2^16, which is more like ~2^20.087. (And that's not even true either, since a couple values like FFFF are forbidden)

Of course, 17 x 2^16 is also a multiple of 2^3:

17 x 2^16 = 17 x 2^13 x 2^3

(reposted/edited because * was interpreted as formatting)

In case hacker news doesn't show emoji, I meant m(

Right, I guess I was thinking more of "not a power of 2" instead of "not a multiple of 8".

On second thought, the author might have meant "Sorry that the exponent is not a multiple of 8" as in Unicode neither has 2^16 nor 2^32 code points.

Agreed; they meant “not a power of 2⁸”.