Not really, they're responding to what they consider an existential threat against them. If you were to ask them to justify the legality of their operation, the argument would presumably be based on self-defense (Article 51), not the NPT.

Iran didn't have to join the NPT, but by voluntarily joining and then essentially failing IAEA inspections (suggesting that they're probably enriching uranium beyond the levels required for any civilian purpose), they've helped Israel make its case for self-defense.