heart arrhythmia isnt always fatal, or immediately pressing/dramatic. i (and many others as it's not uncommon) have premature ventricular contractions (pvcs) which don't really show dramatic symptoms and in low percentages arent that concerning. however, getting to like, 20% or more is when it starts to get concerning as they could tire your cardiac muscles and impede blood flow later in life.
of course, there are various types of arrhythmias and we don't know which one nina had, but i think most people tend to think of something drastic when they hear "arrhythmia" and thus expect dramatic symptoms to be shown.