I believe you're wrong here.
If you modify a hardlinked file, the FS will create new inodes under the hood, leaving existing ones intact. So it's like CoW. At least on ext4.
reply// Didn't double-check, but I've held this belief for over decade now, would be surprised to be proven wrong
Changing a hardlinked file changes it everywhere, unless your specific editor happens to create a new inode. Thus it depends on your editor.
replyCoW is similar except the new inode is created by the file system itself and is independent of the editor.