The version of the tape that was found for the creepypasta wasn't meant to be the full cartoon.
Rather, it's just one scene in the full animated adaptation of Mr Slicker And The Egg Robbers. Specifically, this strip
, where Mickey walks toward the house in which Mr. Slicker is hitting it off with Minnie (who's playing the piano), and then he walks away even more dejected. The mysterious 6-minute cut is where the middle two panels of Mickey actually watching them through the window and realizing how far gone Minnie is would have gone.This is also why the tape is called "Suicide Mouse", despite no suicide taking place in the tape itself; Mr. Slicker and the Egg Robbers is a comic where Mickey does in fact attempt suicide.