I think they mean that rm could check the folder you're trying to delete and see if it has whatever attribute and if so don't delete it's contents. so "rm -rf /" would see that '/' has it and be a no-op, but '~/home/foo' doesn't so it would recurse and start deleting. It could go further do that check for each sub directory recursively, so say '~/home/foo/bar/baz' that 'bar' does have the flag so `rm -rf ~/home/foo/` would delete everything else from the foo folder but leave bar and it's descendants alone.
It does if you ever want rmdir to succeed.
I think they mean that rm could check the folder you're trying to delete and see if it has whatever attribute and if so don't delete it's contents. so "rm -rf /" would see that '/' has it and be a no-op, but '~/home/foo' doesn't so it would recurse and start deleting. It could go further do that check for each sub directory recursively, so say '~/home/foo/bar/baz' that 'bar' does have the flag so `rm -rf ~/home/foo/` would delete everything else from the foo folder but leave bar and it's descendants alone.