Prove that the language is not regular without using Pumping Lemma
1) your proof is correct (and nice).

2)They could mean prove that the complement is not regular (this is a classic result, and the proof is simpler). More generally, if you saw in course some languages that are not regular, you can probably reduce one of them to this one by closure operations.

Another technique without pumping lemma is to show that the number of Myhill-Nerode equivalence classes is infinite.