It encodes two bytes per code point; the encoding of that code point may be longer than two bytes. (Indeed it mentions that it's optimized for UTF-32.) It acc

It encodes two bytes per code point; the encoding of that code point may be longer than two bytes. (Indeed it mentions that it's optimized for UTF-32.) It accomplishes having the full 2^16, while only using safe characters, by not restricting to the BMP. If you read the details of how it was constructed, you can see that the reason the 2^17 version isn't ready yet is because there aren't yet 2^17 safe characters in Unicode! (There are a total of 123,813 characters in Unicode 13 that are judged "safe" by qntm's criteria.)