That sounds like a rumor. Mew already existed in the storyline unless that programmer then added the entire Mew storyline (inc. Mewtwo) - otherwise there was no reason for Mewtwo to exist
The character of Mew was meant to be referenced through the story, but not actually catchable. Considering that you can only get Mew by exploiting bugs in the game, it's pretty believable that it wasn't added above the radar.
