You've understood the general use description of mv (copy-rename-delete), but what OP demonstrates is a situation in which it does something unexpected (and inappropriate) when dealing with mounted directories.
See the part where he creates file 'bar' in mounted directory /mnt, attempts to move /mnt within itself to /mnt/foo, then asks to show the directories containing /mnt.
The result is that the mv fails (so it shouldn't do anything, right?), but still creates an additional directory (/mnt/foo) that now contains 'bar' and has (destructively) altered the fs hierarchy.
However, when he tries it again after unmounting /mnt, the behavior is completely normal for any other file-- mv fails, nothing happens, /mnt/bar hierarchy is preserved, no /mnt/foo created.
See the part where he creates file 'bar' in mounted directory /mnt, attempts to move /mnt within itself to /mnt/foo, then asks to show the directories containing /mnt.
The result is that the mv fails (so it shouldn't do anything, right?), but still creates an additional directory (/mnt/foo) that now contains 'bar' and has (destructively) altered the fs hierarchy.
However, when he tries it again after unmounting /mnt, the behavior is completely normal for any other file-- mv fails, nothing happens, /mnt/bar hierarchy is preserved, no /mnt/foo created.