>>>> The screws add nothing and actually weaken the joint by splintering the tenon.
I would think the screws perform the same function as wedges in that they spread the tenon causing a tighter mating joint.
Again, I could be wrong but it looks to me that there is no adhesive used in the joint. I have been in a couple of Amish workshops and don't recall seeing any air powered nail guns.
I am still of the opinion that the screws driven into the M&T joint are an effective reinforcement just as nails driven into the same location would be.