From the draft:
When evaluating “www.foo.example.com”, the first query would be to
"www.foo.example._bound.com". If the reply to this is “BOUND 0 0 com”,
then the second query would go to “www.foo._bound.example.com”.
My understanding here is that if
www.foo.example._bound.com (which belongs to .com) would reply with NOLOWER, then there’s no way
www.foo.example.com could overwrite this. If the first one wouldn’t reply with NOLOWER, but
example.com would, then the same applies to the labels below