There is a bit in Arsenal I find confusing--a weapon with a pilot upgrade "counts as a drone in all respects", but a weapon with a propulsion system (for which the pilot upgrade is mandatory) "cannot benefit from drone or vehicle upgrades"...

Which is wrong?

Neither. The pilot upgrade allows a weapon to act as if it is a drone. The propulsion system allows the weapon to move but, alone, does not allow it to benefit from drone/vehicle mods. If you apply them both -- so that your weapon can now move and has a Pilot rating -- then your gun is actually a drone, for all intents and purposes.

I think the propulsion system note is there to indicate that that mod does not provide a Pilot rating; the device must be remote controlled by a user, not issued commands.

Thanks!