I was definitely wrong about the "obvious" part, then.  The 'if __debug__:' idiom is completely optimized away by -O, just like asserts are.  That is, in -O mode, there is no if test, the code block is just omitted entirely.

Having said that, though, I can't find it documented other than by the implicit equivalence given in the assert docs.

Hmm.  Yes, it does seem to be a CPython implementation detail.  See issue 8379.  Assert being removed is also an implementation detail, though (see the reference to "the current code generator..." in the assert docs).  I suppose that '-O' itself is a CPython implementation detail, in a sense :)