^ they are not the same.

as I said in my very first reply, SingleEmailMessage is class within another class called "Messaging".

A class within a class does not necessarily mean that the inner class (SingleEmailMessage) is inherited/derived from the parent (Messaging) class. As I look at it, SingleEmailMessage is not inherited from Messaging class, because SingleEmailMessage does not posses the methods of the Messaging class. This in turn corroborates to the documentation that Messaging.SingleEmailMessage and Messaging.MassEmailMessage were inherited/derived from Messaging.Email base class.

to illustrate, they would look much like this in the Apex Code:

public class Messaging {
	public virtual class Email { // the base email class
		//common methods
	}
	
	public class SingleEmailMessage extends Email {
		//inherits common methods
		
		//SingleEmailMessage specific methods
	}
	
	public class MassEmailMessage  extends Email {
		//inherits common methods
		
		//MassEmailMessage specific methods
	}
}

P.S. you cannot even define a variable/object of type Messaging (i.e. Messaing msg = new Messaging()), so as I previously mentioned, Messaging resembles much of a namespace, rather than a class.