They're wrong because the clock premise is entirely contrived and subjective.
That's an opinion not a proof, just like the setting of the clock is an opinion. Who's more likely to be wrong on this point, people studying the issue or a knee-jerk response because they're "liberal"?
Konrad,
One problem is that this "clock" has gone from trying to assess the risk of global nuclear/thermonuclear war, to a whole range of other possibilities as well, while most of the scientists connected with the Bulletin of the Atomic Scientists are theoretical physicists. Some of these other issues which have been added would appear to be outside the area of their professional expertise. The implication of that is that this is informed speculation, at best, and guessing based on a political agenda, at worst. I see nothing in that, that would be better informed than the speculation of any other reasonably well-educated person.
As for reality, yes, there are any number of scenarios for the destruction of civilization, some natural, some man-made, and some (like a pandemic) which could be either. The natural ones (asteroid strike, EMP from a massive solar flair, supervolcano eruption, and possibly climate change), we can do little about, in any case. The same may be true of a naturally occurring pandemic, which is at least as likely as one caused by, say, bio-terrorism. Life is fragile, human civilization is a tenuous affair, and neither comes with a guarantee. Even IF we could magically get rid of all the nukes tomorrow, and IF we could magically get rid of all "greenhouse emissions" tomorrow, there is absolutely no guarantee that nature won't get us, or at least most of us, eventually. No one gets out of this world alive, so everyone might as well make the best contingency plan they can for any event which might be survivable, and then enjoy today; no one can guarantee ANY of us tomorrow.