They probably concluded it wasn't possible in this case. From their FAQ: [Q] Is there a way to detect Downfall attacks? [A] It is not easy. Downfall execution

They probably concluded it wasn't possible in this case. From their FAQ: [Q] Is there a way to detect Downfall attacks? [A] It is not easy. Downfall execution looks mostly like benign applications. Theoretically, one could develop a detection system that uses hardware performance counters to detect abnormal behaviors like exessive cache misses. However, off-the-shelf Antivirus software cannot detect this attack.