Taking a peek at pp.rb, #pp is defined for several things, including Kernel.
Kernel methods get included in everything, which means everything gets an
instance method #pp.
I'm guessing that #method_defined? tells you if objects of the given Module
have a given instance method. #sharing_detection is an accessor for the
class instance variable @sharing_detection of PP, defined as:
class << PP
Or some such. This means that the class in which the method is defined is
the singleton class of PP, accessed by:
class << PP; self; end
Sure enough, if you call method_defined? on the singleton class, you get
PP responds to #sharing_detection, but it's defined in its singleton class.
PP defines a method #pp, so objects of class PP will respond to #pp.
Kernel defines a method #pp so objects including Kernel will respond to #pp.