Whether 78% is mandatory in Engineering to get a call from IIM Ahmedabad?

Yes, there have been mentions of having at least 78% in Engineering for general candidates for admission at IIM Ahmedabad. They may have created the shortlist based on these criteria. The thing is that most of the IIMs want to improve their diversity in the batches as the majority of the students who apply and eventually convert to IIM Ahmedabad are engineering graduates.

So they keep such informal cutoffs to increase diversity. But unfortunately, due to such criteria, many bright students are kept out of the process. Many universities also have toppers who have scored just 65%. But to keep in mind, it is not a mandatory criterion.