I figured out what was going on with my "duck" at MUSSOL Restaurante!
The title of the dish said "magrete de pato" but since the description only said "pechuga de pato" (breast of duck), I brushed aside the word "magrete" because I assumed it was a Catalan word that I did not know. The truth is, I did not know the word, but it's not just a Catalan one.
Thanks to a few sites, I now know that a duck magret is indeed a duck breast, but that of a duck that has been fattened for foie gras. Since the duck is force-fed a diet that is high in corn, the pectoral muscles become stronger and the flesh has a less-tender texture, but the taste as well is decidedly different than traditional duck breast. Hence, the confusion in my taste buds.
Check out my sources here and here.