I found this part interesting:

> Notice that when the key is, say, 40 bytes long, it’s padded with zeros to make it 64 bytes long. So the actual entropy of that key is equivalent to the 40-byte long key. But if the key is longer than 64 bytes, it’s effectively being shortened to 20 bytes. So its entropy is lower. If you want to maximize the complexity of the key, and you’re the one choosing the key, go for something around, but never more than, 64 bytes.

Perhaps that's why some sites have a maximum character limit for passwords. (Although I suspect many are just doing it wrong.)