False Gods book question

Spoilers!!!

When Hektor Varvaras is killed during the final battle with the Brotherhood, Loken assumes it was a bolter round that caused the death of Varvaras. Because his chest had exploded.

Later, when Torgaddon reveals that the bolter fragments from Varavaras were examined and came from an Astartes bolter, Loken is surprised.

Until now, i always thought only Astartes or at least Imperium troops used bolters. Did the Auretian Technocracy Brotherhood or other enemies during the Horus Heresy use bolters aswell?

Please no spoilers for books after False Gods! :)