The plate should work on its own. The thing that stands out to me is there's not even deformation. None of the impact is being caught and spread out. Just perfect holes. So yeah. Well outside the envelope for where the kevlar shell would have made a difference.

You might be thinking of the anti-spalling coating on the front. Not exactly required but helpful.

That layer traps the prices of the splattered bullet after it impacts the plate. Without it those pieces can hit your chin or arms.

OR you're thinking of ceramic plates? Ceramic breaks up the bullet while the Kevlar behind it catches the slowed and broken pieces.

At least for steel plate, it should work without any supporting material.

Kevlar around it would help catch spalling from a bullet flattening out and spraying little bits of shrapnel out across the surface, but since these ones aren't actually stopping the bullets, there is no spalling to catch.