I think there's an argument to be made that the way an API is implemented is an implicit contract that ought to be upheld.
That sounds wrong to me. If your consumers have to rely on assumptions beyond what's provided in the contract of the API, your API is leaky and/or broken.
[edit] I understand the pragmatism necessary in the case of the glibc issue, but to clarify my point I disagree with the general assertion I'm quoting.