What does "reasonably safeguarded" mean?
I don't believe there is an universal definition on this or AMEX has ever defined in its terms.
But FWIW - I would agree with AMEX that your bike was not reasonably safeguarded. In fact, I would say it was not properly to the extreme. At the minimum, rack or no rack, I would expect that you would lock your bike against a fixed structure or your car.
BTW - I see that you are from San Francisco. Because of the high rate of bike theft in the area, I would say you did not do enough to protect your bike.
I would assume that the locked garage would have been reasonable enough, but Amex seems to disagree.
If you store in a locked garage that only you have access, then I agree. Just because it can access by residents only, it does not mean the thief is not one of the residents or the bike was removed by the management. Your bike was in fact "parked" in a common area.
You may be able to do so. At the minimum, I know that mandatory binding arbitration is a known option for AMEX to resolve problems.
But I would not expect anything.