Well, when it was instituted, Nuclear Armageddon was
the existential threat to humanity, and therefore that was the metric they chose. Everything else paled in comparison. They weren’t hugely aware of the threat that things like climate change, or trophic cascades, could present. They were pure hypotheticals.
They didn’t, however, call it the “Nuclear Armageddon” clock, rather the doomsday clock, because the point was tracking that overarching threat to the continuation of the human species.
As we have now recognised that there are other categories of threat which are potentially civilisation-ending, we, in my opinion rightly, include those in the metrics of the probability of doom.
Climate change is like having a slow growing but untreatable cancer. It’s going to get you, unless something unexpected and drastic happens. It may even get you sooner than you expect - right now we are tracking the upper bounds of every model, and the worst case scenarios are consistently getting worse.
So, nay, it no longer tracks the sole threat of nuclear Armageddon, but I think the estimation of how close we are to midnight is pretty accurate.