The use of the adverb "securely" is a bit unfortunate. As you point out, ext4 can't control the behavior of the underlying hardware, which means that it can't guarantee that the deletion is actually secure. You might have a SCSI bus analyzer that records every write sent to the drive, in which case ext4 tells you that the deletion is "secure" ... but someone else can easily reconstruct the file.
A better name would be "make this file difficult to recover or undelete at the /dev/sdX block device interface level" delete, but that's clunky. I suspect that this attribute is sort of an opposite to chattr +u, if you will.