So? Sending email is free; you can just send the email without knowing if the address is correct or not. Change the "| md5sum" to "| xargs mail" and you don't need the Gravatar hash anymore.
Say, you suspect that Alice on stack overflow bad-mouthing software vendor "HAL", claiming impartiality, is really Eve, working for competing software vendor "Moon", you might be able to confirm that by using your knowledge of moon.com e-mail addresses, and checking the hash of eve@moon.com, causing SO to breach the privacy Alice/Eve expected when signing up.
Technically, yes, this article is correct. The Gravatar FAQ even discusses this issue, IIRC. But in practical usage, I can't imagine how this would prove important.