The strip is not stiff in bending or compression, only tension. Therefore it is not a batten, it is soft like the surrounding sail.

It take the tension rather than the surrounding clothe so the clothe doesn't stretch.

To understand the forces on the top batten, imagine there is no clothe; only the batten and leech tape (including the top leech tape). Then pull down on the trailing edge leech tape and you will see the batten is compressed.

So instead of this force travelling around the leech tape and compressing the top batten (which will push depth into this part of the sail), it is predominantly bypassed along the reinforcement that is shown in the photo, leaving the top batten with its pre-compression only.