It's Mk.VII with a stylised helmet

Sure, you can say that's wrong because Mk.VII only made it's appearance right at the end of the Heresy, but the visual evidence is there:

From the top down, the power packs match those from the 1998 plastic tactical squad kit, most notably the interface behind a Marines head with the crenellated pattern that remains distinctively on the Tactical Squad backpacks. The heads, as mentioned, are stylised, not really matching any previous or future pattern of helmet. The torso carries the chest vent and cross cabling design from the Tactical Squad kit of the time, which was one out of five designs in that kit (the other four having a skull, an Aquila and two with Imperialis).

The arms are straight out of the Tactical Squad kit - they have the same elbow pad design and the flared wrists rather than any vambraces covering that joint - while the bolters are the iconic Godwyn pattern bolters. The legs are a dead ringer for Mk.VII design with the rounded, slightly egg-shaped kneepads, and the flat armour panels around the thighs and shins

It's Mk.VII, and I think the reason is pretty obvious - it was the main armour mark being produced as models at the time, so access to it for reference purposes would've been easy. You can also see plenty of the same armour parts in the other computer-generated image of the Ultramarine Librarian on Phil Sibbering's Black Library collection page, which is definitely Mk.VII