All Infernals are heroes.
It's a prerequisite for becoming an Infernal. You must be heroic.
Also: no, not all Infernals were wusses who chickened out at their moment of glory. Some actually went through with it, and, had there been a Solar Exaltation available, would have Exalted and succeeded.
But there wasn't an Exaltation. Because it was stuck inside a demon at the time. So they failed.