It's kklog k. Just because the min heap has a capacity of k doesn't mean you are only iterating over k elements when adding to it. You are generating combinations of pairs from n1 and n2 and then adding each one one of them to the heap. Assuming worst case where n1.length == n2.length then it's kk which gives the time complexity of k^2. For each of those you add it to the heap which is log n therefore you have kk*log k.
Find K Pairs with Smallest Sums