As far as bio doom being easily defensible, I think the important point to make is that it really doesn't matter by what action or method superintelligence chooses to wipe out humanity with, it will likely be unthinkable because it would go about solving problems in far more efficient and indeterminate ways than humans would. The authors are using bio doom because it's a method, and one that's easy to imagine. To ask them to come up with a likely method that superintelligence would use would be asking them to think like a superintelligence, which they clearly can't.
All told, I’d estimate between 7 days and 3 months for an AGI to have come up with a reasonably good system of ethics that’s been approved for deployment, perhaps in stages.
As far as bio doom being easily defensible, I think the important point to make is that it really doesn't matter by what action or method superintelligence chooses to wipe out humanity with, it will likely be unthinkable because it would go about solving problems in far more efficient and indeterminate ways than humans would. The authors are using bio doom because it's a method, and one that's easy to imagine. To ask them to come up with a likely method that superintelligence would use would be asking them to think like a superintelligence, which they clearly can't.