Not my field, but from a quick look at the methodology of that paper it's not particularly compelling by itself. It's based on a symptom survey, so they're showing some similarity between long Covid and MCAS patients. But a lot of the symptoms of both syndromes are really broad, like brain fog, muscle pain, fatigue, etc. So I'm not sure how compelling similarity of symptoms is in terms of indicating underlying causes there. Certainly worth following though.
I did notice that one of the symptoms they included in that paper was literally 'Fasting worsens symptoms', with significant increased symptom severity in that category for both long Covid and MCAS patients compared to controls and pre-long Covid patients. So while that won't be the case for everyone (it's an average, so individuals may vary substantially), that does suggest fasting won't be the way to go generally for that particular condition.