Not part of the language as far as I can see, more a way how you do this in python
In a comment here on HN. the author clarified. What he means is that a whole bunch of builtin functions operate by calling methods on the objects they are passed.
int(n) --> n.__int__()
format(x, spec) --> x.__format__(spec)
It's not multimethods. It's just calling methods on the objects you are passed.