Question: How do you get a PGM armed AC to use its other weapons?

[Imp]

I am not sure how it works but use of weapons other than the cannon is a bit hit and miss from my observations.
Probably has to do with unit class and weapons though in your case ihe had his eyes closed, received no target lock and somehow missed the targets of opportunity to.
Pilot makes a judgment call I guess on whether it’s worth using his payload, versus infantry he may not if armour orientated.
If it had seen higher value armour in the area or an AA asset it might have dumped everything on them. Sometimes it will target them directly or make a secondary offcentre attack against them. You can semi predict but that’s all with practice.