It's mainly about intention, not coincidence. You did not kill those extinct animals, nor did you ask or pay someone else to do it. It was not your nor someone elses intention that was involved in their extinction. It could still be seen as exploiting one of earths limited resources though, but that's not directly related to veganism.
I know some vegans consider it fine to consume animal products that did not cause harm or are not exploitative, or even meat from animals which were not killed or harmed intentionally.