but then we get into other questions such as "what is an acceptable level of bias in an experiment where you perform it some number of times, be that 1000, or more?"
When considering generating private keys for bitcoin, then my answer is zero. I don't see why you would settle for anything less. This is why I advocate for using coin flips with von Neumann's algorithm, since by doing this you can be certain you have eliminated any bias in your coin, as well as not introduced any new bias by performing randomness extraction or other processes you don't fully understand on your data.Any method of testing for bias can never rule out bias 100%, only make it less and less likely but after an exponential number of test flips/rolls.
what's even more horrendous is how no one ever called them out on it until people started losing money. 
it's not like they were hiding the insecure code. apparently it was sitting there right on github for all to see. but no one did.
People involved in bitcoin who have the ability to read and analyze code, as well as the time and motivation to do so for free, generally aren't using random low quality wallets like Cake they stumble across on the app store, which might explain why nobody picked it up sooner. it's not like they were hiding the insecure code. apparently it was sitting there right on github for all to see. but no one did.