https://en.wikipedia.org/wiki/Magnus_effect#In_aviation

(can't help but think "tilt rotor" of a different kind)

So I am reasoning with a side wind 'on the beam' you get a force pusing forward. Which incidentally is a traditional rigs best point of sail. This doesn't appear to be the full story, since the boat in the photograph is healed over in the same way as the gaff rigged sailboat next to it, so there appears to be some sideways force too. Laterall resistance of the hull in the water will force that into a resultant force that is mostly forwards (but with some leeway) I presume.

I am supossing that when running before the wind this means the force is to one or other sides of the ships axis (depending on the rotation). The resistance of the hull to lateral motion forces this again into forward motion. But the sail is indeed being obstructed by the containers and I believe the force will therefore be diminished as per the OP, since the wind has led sail area to act on.

I am inferring all this since the wiki is light on detail.

I would be concerned that one cannot reef this kind of sail. In a storm it is going to heel the boat more, change its motion through the waves, perhaps press the bow down. Bearing in mind that on a traditional square rigger, one would lower the top part of the mast before a storm to reduce windage.

I feel a better reference would be an aid... but I'm with the op, at some points of sail the containers are in the way