Zach says that Ellen hated him ever since he messed up a line on her show 10 years ago.
Ellen Degeneres did have her own sitcom based loosely on her stand up act. It was a hit show until her character 'came out' and she lost her sponsors and the show was cancelled. Her career had just begun a tremendous upswing at the time of this show, and her talk show contributed greatly to that. Matt LeBlanc asked her to guest star during a real interview on her show, and she agreed. The actor who played Zach was never actually on Ellen's sitcom.