Thank you for that link! It's very interesting to me that all the calculations laid out here assume independent failure events.

It is universal practice within cloud service providers to span redundancies across "fault domains" - basically things which could make failures correlated, like being in the same machine/power strip/datacenter/geo region. If you assume your fault domain analysis is good, then your failures should be independent. Many global outages are the result of a previously-unidentified fault domain, like the Azure certificate issues. Of course past a certain point it becomes unimportant - who cares about your data if an asteroid takes out every datacenter on Earth at once?