I posted a similar question not too long ago, and I still do not understand why this
doesn't work.

If you look up 'Class Member Protection and Hiding' in the help, it states:

Properties and methods in a class definition are Public by default: code
in other classes or procedures can set the properties or call the methods. 
Properties and methods that you designate as Protected can be accessed only 
by other methods in the class definition or in subclasses of the class. 
Properties and methods designated as Hidden can be accessed only by other 
members in the class definition. Subclasses of the class cannot "see" or 
reference hidden members.

So then, since everything below is part of the 'Class Definition', why then does
this not work?

The help text above says "Properties and methods designated as Hidden can be
accessed only by other members in the class definition"

The button is a member of the class definition, and should be able to
call the hidden method on the form.

oForm = CREATEOBJECT('frmMyForm')
oForm.Show(1)


DEFINE CLASS frmMyForm AS Form

	AutoCenter = .T.
	

	ADD OBJECT cmdOk AS CommandButton;
            WITH Caption = 'Test'
      

	PROCEDURE cmdOk.Click
      
    	    This.Parent.Hidden_Method()
    
        ENDPROC

	PROCEDURE cmdOk.Init()
		
		WITH This
		
		    .Top = 10
		    .Left = 10
		    .Height = 25
		    .Width = 75
	
		ENDWITH
		
	ENDPROC

	
	
	HIDDEN PROCEDURE Hidden_Method()
	
	    MESSAGEBOX('Here we are')
	
	ENDPROC
	

ENDDEFINE