They do not take rats from say a maze solving study, then give them diabetes for a different study, then give them a brain tumor before putting them in the decapicone (a real product).
I figured, but in the meme story this is pretty explicitly what is happening.
The diet experiment is presented as present tense. The cat smell experiment is described with “I once ran an experiment”, part of the speaker’s “thesis”, which is in the past. They are clearly keeping #42 alive to be used in totally separate research, in this fictional The Onion esque scenario.