I think it didn't just deleted his personal files; it corrupted the installation itself. It might be another bug. I'm not sure though.

Searching around for some sort of source, I found this article[1], which says "...some of my readers told me their hard drives were corrupted and thus were unable to roll back the update." Of course this may be hearsay and false information in times of panic, so I take the statement with a grain of salt.

[1]https://www.forbes.com/sites/jasonevangelho/2018/10/06/micro...