No, it is not a coincidence. I made some investigations and ready to tell all in order. First, the final song or rather music video, a parody of the very famous video for the song "And I Swear" group "All 4 one". Anyone will be able to watch it from the link at the end of the article.
In the second phase of the history of the creators of the cartoon convert "And I Swear" (and I promise) in the consonant, but it is still English "Underware", which is already marked as "underwear".
In the preparation of sound tracks for domestic rentals, "underwear" is transformed into "boxers".