А не, оказца наоборот, смотрите
c =
C.newc.singleton_class.__send__ :include, M
p c.singleton_class.ancestors
C.__send__ :include, M
p C.new.singleton_class.ancestors
выведет:
[#<Class:#<C:0x00000002317110», M, C, Object, Kernel, BasicObject]
[#<Class:#<C:0x00000002316cb0», C, M, Object, Kernel, BasicObject]