Although it seems unlikely x-risk reduction is the best buy from the lights of a person-affecting view (we should be suspicious if it were), given ~$10000 per life year compares unfavourably to best global health interventions, it is still a good buy
although it seems unlikely that x-risk reduction would be the best buy by the lights of a person affecting view, this would not be wildly outlandish.
Thanks for linking; in particular, Greg covered some similar ground to this post in The person-affecting value of existential risk reduction: