The correct result of the above query should be an empty set. Here is an explanation:
Both rows from T2 satisfies the correlated predicate T1.C2 >= T2.C2 when T1.C1 = 1 so both rows needs to be processed in the same group of the aggregation process in the subquery. The result of the aggregation yields MAX(T2.C1) as 2. Therefore, the result of the evaluation of the predicate T1.C1 (which is 1) IN MAX(T2.C1) (which is 2) should be an empty set.