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."