I tried what I said before and it worked. Are you thinking of something different? What they were talking about in that thread was parabola movement, not 'recoil'.
Here's an example:
on mouse clicked:
・ create object "cannonball"
・ apply impulse 1 to cannonball at angle(cannon.x,cannon.y,mouse.x,mouse.y)
・ apply impulse 1 to cannon at angle(cannon.x,cannon.y,mouse.x,mouse.y) + 180
This works with some changes to the mass of both objects.