The wrong part is that Let's Encrypt was willing to issue a valid cert to anyone who can temporarily redirect traffic. The authorization should have been done better, for example, sending a certificate to operator's email.
There is no such thing as an "operator's email". Over time there has been a wild growth of webmaster@, admin[istrator]@, root@, postmaster@ and so on, but having access to them proves very little. Some email operators just aren't very restrictive with their allowed usernames, and that's before we get into the corporate world where the first-line helpdesk person weeding out the email received on that address probably isn't supposed to issue certificates!
This method has been (mostly?) banned for a reason, see for example CA/B's ballot SC080v3.
This method has been (mostly?) banned for a reason, see for example CA/B's ballot SC080v3.