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> "the subclass provides" doesn't actually imply anything about overriding, I think.
Yes, that was the thrust of one of the SO answers. Unfortunately, that explanation doesn't work for arithmetic operators, though: there an explicit override is necessary. Here's another example, partly to get away from the extra complication of __eq__ being its own inverse. After:
 class A(object):
 def __lt__(self, other): return True
 def __gt__(self, other): return False
 def __add__(self, other): return 1729
 def __radd__(self, other): return 42
 class B(A): pass
we get:
 >>> A() + B()
 1729
 >>> A() < B()
 False
So the addition is calling the usual __add__ method first (the special exception in the docs doesn't apply: while B *is* a subclass of A, it doesn't *override* A's __radd__ method). But the comparison is (surprisingly) calling the __gt__ method first.
So we've got two different rules being followed: one for arithmetic operators, and a different one for comparisons.
This isn't a big deal behaviour-wise: I'm certainly not advocating a behaviour change here. But it would be nice to document it.