Actually, it looks like it's the "Spawn" flag. Removing the "Remote Detonate" flag didn't help, but when I removed the "Spawn Cluster Bomb Baby" part, it acted like a regular missile. I guess we can't have flak with submunitions.
I also tried giving standard flak "Spawn Cluster Bomb Baby, 5" which was funny because the Hecate took itself down below 50% hull in a few seconds.
Edit: Also, if you're looking for something weighty for the Hecate to shoot, Infyrnos are way too small. You'd need the Helios model at least for it to be barely convincing.
I tried giving a beam a shockwave, but nothing happened. Although I guess if it's meant to be a powerful beam then you could just FRED it in.