To be fair, MkVII is introduced before the end of the Heresy by the loyalist forces, when they evacuate armour researchers from Mars and continue development, so that picture could still be accurate if it is set very late in the Heresy. The helmet of the Marine in the foreground is a MkVII helmet not a MkV helmet (due to the exposed cabling of the respirator) so there isn't really any other explanation for it.
The White Dwarf article actually says that MkV suits were newly produced, and that the old parts used were components such as cabling, not entire sections of armour, like legs or arms. It says the armour plating itself was produced by the Techmarine artificers using whatever they had to hand and reinforced with the bonding studs, so it never described as just being a mismatch of different armour patterns, but more as a rough and ready group of similar designs using whatever components they could easily produce or were readily available but with improvised armour plating over the top.I would have liked it if they'd taken the opportunity to make the models to fit with the current (which has been around quite a while) background to it after all they'd only ever made one of the WD 129 MKVs as it were.
I probably fit into the same category at the Forge World chappie Alaric mentions.