Is afib really stress in disguise?

No: No, but stress is one of the many things that can precipitate it in an already diseased heart.

Not really: Atrial fibrillation has a long list of possible causes including hyperthyroidism; heart attacks; congenital heart problems; heart valve problems; stimulants like caffeine, tobacco, and medications; viral infections; stress from surgery, pneumonia, or other illnesses; sleep apnea; sick sinus syndrome (the heart's natural pacemaker); hypertension; and alcohol drinking binges. Stress can exacerbate.