Admin's reply has to be the best I have ever seen.
My best guess is that the legacy extension has been manually compiled in PHP 7 setup on the server, because I tested it myself and the same thing happens... I wonder why this was?
I wouldn't be surprised if this user has uploaded a nulled install of XenForo anyhow... it's relatively rare that a user would have resources to pay for a forum software but not hosting or domain...