"Subject to availability" is pretty broad. Is it like a mall parking lot where is genuinely shocked if you cant find a spot? ("Once in a blue moon" was the statement, I believe?) Or is it more like a lottery ticket?
It's also not what I expect Amex to lead off with, especially as "world-leading customer service" is part of their brand image. They could have said "We understand crowding is an issue, particularly at SFO at peak times. Because of this, we are adjusting our access policies, such as limiting it to 3 hours before flight departure for originating passengers. We're very sorry that we were unable to accommodate you on your recent visit, and feedback from valued cardmembers like yourself that helps us inform these changes". That would have cost them exactly zero, and it would have left a far better taste in my mouth than saying that technically, they don't have to provide anything at all.
In the words of Ted Knight, "you'll get nothing - and like it!"