Neither is North Korea, but people still claim (justifiably) that North Korea is in defiance to the treaty.

The NPT is de-facto a universal treaty, any UN member who is not a member is in defiance even though de-jure they are not. Simply put, what I am saying is, Israel—along with North Korea, India, and Pakistan—are going against the expectation of the rest of the world when it comes to nuclear non-proliferation. They are in defiance with the nuclear norms of the international community.

Note that people also said the USA was in defiance to climate change treaties when they failed to ratify Kyoto, and when they withdrew from Paris.

I wouldn't say such a universal expectation exists; NPT itself has a mechanism for states to withdraw based on "the supreme interests of its country". Countries with generally unthreatening neighbors might be expected to sign; Israel certainly doesn't fall into that category.

So Israel is enforcing a treaty they're not a part to?

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.

Using this argument Palestine Syria Egypt Lebanon Turkey Iraq Iran all need nukes since they face existential threat from israel