They sometimes don't. The article even acknowledges this: > Copy commands and even deletions can fail I've had that happen too many times, so I don't know why

They sometimes don't. The article even acknowledges this: > Copy commands and even deletions can fail I've had that happen too many times, so I don't know why would fill up my disk with a hacky spacer file, which surely can also fail to be deleted when the disk is already full.