Each of the variants, including the AIM-9X, are listed on https://en.wikipedia.org/wiki/AIM-9_Sidewinder as "IR/Laser fuze".

Boresight shot towards a slow, non-maneuvering target plus laser fuze should be just fine.

What do you mean by boresight shot? Even in BORE mode, the AIM-9X seeker tracks the IR signature of the target.

Arm missile, fire straight ahead, let proximity fuze do the work. AIM-9X has demonstrated capability to fire without a lock.

I think you are confused about the difference between fuzing and seeking.

There is no publicly known way to fire an AIM-9X without seeking. It can be fired with out the seeker having a lock and given updated guidance from the launch platform via datalink to direct it towards the target so that it can then acquire the target with its own seeker. Even in BORE mode, the seeker attempts to acquire the target immediately.

The fuze is a set of two separate mechanisms, an active IR laser and photodiodes for the non-contact fuze and a contact fuze.

Perhaps they painted the target with an IR laser. Just a guess.