I can see arguments either way. [On the one hand, he literally was no one special. There were thousands, tens of thousands, of other Astartes, of greater renown. On the other, was he a representative of his kind? Was he alone being judged, or the entire Adeptus Astartes? ] I find their 'reason' suspicious as well.

A cogitator is a repurposed human skull used to power electrical communications.