I find this to be like saying to someone with cancer "Don't bother with treatment, you aren't dead yet". A bucket list is for plans and actions, not attributes inherent to existing in the first place.

Other commenters have said that it is more about things you may not have done without having it on the bucket list for a reminder or incentive. In this case, we can reasonably expect Gates meant putting effort into avoiding death, not "I was immortal, but now feel like trying to win the Hardcore Mode Bucket List challenge.