Looked like the mall to me. Logically, where else would the large group in that scene gather at that time?

Perhaps part of the confusion results from the many sets that were part of what was portrayed as the Corleone compound. That Staten Island mansion's exteriors represented the mall during Connie's wedding, when Michael came back from the hotel (and later the hospital), and when Kay tried to drop off her letter to Michael with Tom. But the interiors, I believe, were sets, and some other outdoors (like the scene you cited and when Carlo gets killed) may have been shot elsewhere than the Staten Island mansion.