I believe AWS secrets are 240 bits. That is a pretty massive space. I don't know how many active secrets are out there, but I think someone would need to get very lucky to collide before the attack was noticed and stopped.
Other partner's secrets may be more susceptible.
Edit: I did not consider the paired access key which is another 70 or so bits. I think you'd need to collide on both to make someone have a bad day.
