Yup. An ATP is not required for 91 operations. The issue is not the size or type of aircraft, but rather that of operating scheduled service to the unsuspecting public.

A 30-seat turboprop flown under 121 DOES require an ATP.

A 747 could be flown by a (type rated) private pilot if he was doing it for fun.

An exception mught be if you work for a 121 carrier and conduct 91 operations under their OPSPEC.