Barbed Devil as grappler vs. Medium PCs

I’m preparing an encounter with devils, and I was reading the Barbed Devil entry on the MM.

It’s stated that it has an Impale special attack while grappling, Improved Grab and Improved Grapple.

It’s clear that its purpose is to latch onto an enemy ’till they’re dead, however, Improved Grab doesn’t really work on PCs (as they are of the same size), and the devil is missing Improved Unarmed Strike (prerequisite for Improved Grapple).

Improved Grapple isn’t marked as bonus either, so it doesn’t work without the prerequisite.

How can I fix this without changing the CR too much?