Is my comment not clear? They are not "readline shortcuts"; readline got them from Unix. GNU targeted Unix as the system to replace with free software and adopted its conventions.

Beside Ctrl-W and Ctrl-U, another convention from the Unix TTY found in readline is Ctrl-D.

getpass doesn't support editing at all; it runs the TTY in "cooked mode", just with echo disabled. The operating system kernel implements the rudimentary editing (Ctrl-W and Ctrl-U).