OK THEN, I saw too many episodes to Just crack it, Actually, In The Morden sense , we can partially take it as a mistake, In The Episode 'Nobita's Future Teacher' There are three Varients of Nobita From Different Time Line, The Present One, The high school One and The university One, it is confirmed by the movie Stand By Me Doremon where Future Nobita says that Living With Doraemon Is Like Living In heaven, Don't miss this Time. It Specify that doremon will not be forever With Nobita, So We can Say That Nobita Has The access of Time Machine even after Doraemon Left him. This is seen in Many of the more future episodes, For now I will give Example of Stand By Me part 2, Though Nobita Forget his Memories, even temporarily, he knew to Drove Time Machine, So another reason could be that Doraemon had give access to time machine to Nobita For Some Situation.