I didn't understand this either (and I'm still a little fuzzy on it). Check out http://www.spanishdict.com/translate/el%20mismo and http://www.spanishdict.com/answers/179921/what-is-the-difference-between-el-mismo-and-lo-mismo.
Think of "lo" as the pronoun, with "mismo" as its adjective: "the same thing." In this case, "the" apparently doesn't need to be stated.
I would expect "el mismo/la misma" to be accepted in this instance, as well. Spanishdict.com gives the example, "El pueblo ya no era el mismo" = "The town was no longer the same."
"He is the same." He is unchanged. Adjective = el mismo or la misma
"I'll have the same." I'll have the same (thing). Pronoun = lo mismo or la misma
But I still have a question:
"That looks like a dog to me" "It is definitely not the same (thing)" (pronoun) = Definitivamente no es lo mismo
"Does that dog look different?" "It is definitely not the same" (adjective). Shouldnt' this be Definitivamente no es el mismo? In which case, both lo and el should be acceptable answers for this question depending on context.