It is if the alarmists have their way and we commit trillions of dollars in resources to CO2 mitigation. That's trillions of dollars that can't be spent to create wealth and bring people out of poverty. That's a lot of opportunity cost.
To be clear, YCombinator's money is theirs and they can invest it however they choose. I don't know what fraction of their total investment will end up being committed to this. My objection is to the "existential threat" language, which makes it seem as though they would be willing to drop everything else and commit all of their resources to this effort. (After all, if it really were an existential threat, why wouldn't they? How could they justify funding any other startups?)