The title here is simply wrong. According to the details in the post the 0day vulnerability is in IE8, not Flash. Flash was used to heap spray, maybe because the vulnerability was in the ActiveX layer, or maybe because IE8 doesn't have a JIT and Flash's JIT spraying mitigations don't actually work. Either way none of this constitutes a 0day in Flash because ActiveX would be IE's bug and Flash's utility for heap spraying is already very well known.