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In this case, the concrete class is the one missing a method. 
Concrete classes are allowed to provide more features than the corresponding ABC, but the converse is not true to the best of my knowledge.
dict_keys .register()s as supporting the Set ABC, so it does not automatically pick up the method through inheritance. Put another way:
>>> # dict_keys provides the Set ABC API
>>> isinstance({}.keys(), collections.Set)
True
>>> # The Set ABC provides isdisjoint
>>> hasattr(collections.Set, 'isdisjoint') 
True
>>> # Ergo, dict_keys should provide isdisjoint ... but it doesn't
>>> hasattr({}.keys(), 'isdisjoint') 
False
See also Issue9213 for another case where a concrete class is missing a method provided by an ABC it claims to support.
I sort of wonder if .register() should verify that the concrete class provides all of the methods of the ABC.