The 2010 S3 calculation is obviously also wrong! I totally feel your pain in terms of wanting to have a directly comparable answer, but the reasons you give that "it doesn't matter" (and others, including correlated faults, software bugs, model risk, and security breaches) are actually reasons why the stated durability number is wrong. Honestly IMO a probability of 1-10^-11 is the wrong answer to pretty much any question; model risk is going to dominate that for any problem more complex than 1+1=2.
That said, although neither your system nor Amazon's should be expected to have anywhere near "eleven nines" of durability in reality, if as I understand it S3 is split across availability zones in a region and your product has all splits in a monolithic DC, I would expect S3 to come out ahead in a more careful analysis. (But note that S3 is not really a seamless multi region product, though there is an option to set up cross region replication.)