Trying to get a Symlink but it turns to be an Alias
The command you used is ok. This created a link in ~/Library/"Application Support"/MobileSync that points to /MacintoshHD/Backup. And this is is a symbolic link.

However, is you get the info on Finder it will catalog as Kind: Alias.

The first letter of the output of ls -l will show you that is a symbolic link:

lrwxr-xr-x   1 username staff    8 Dec 10 18:27 MobileSync -&gt; /MacintoshHD/Backup/