You're confusing the irrelevance of the word with its meaning. The word fetch did not catch on (which was the joke), but the word did not mean irrelevant, it meant “cool” or something in that vein.
Rachel McAdams finally snapped at her that the word was not going to catch on and that she should just stop. Tears were then shed.
No, they're claiming that "fetch" is interchangeable with "irrelevant" according to the movie.
This is clearly not stating that the actions by both parties are "irrelevant", it's clearly slang for "cool" or "awesome". The movie was alluding to the word itself being irrelevant in its use, not the word meaning or being synonymous with "irrelevant". I hope this cleared things up for you.