To say Google is not the place to look for reliability is a pretty comical statement.
99.99999 is 3 seconds of downtime/year.
As you get beyond 5 nines, environmental factors begin to dominate (if the service is networked: network unavailability, if the service is onsite, power outages and weather) any reliability inherent in the service.
If you are into reliability you really shouldn't take Google seriously.
The argument is basically the following:
(1) Consider the claim that getting to 100% reliability is important.
(2) If the claim were true, then it would be important to go from 99.999999999999999% availability to 100%.
(3) That is obviously not important, so the claim must be false.
(4) Since we have shown that the best target value is not 100%, it must be some number x, with x < 100%.