Sweet Magos Biologis.
I wonder if he's Adeptus or Dark Mechanicus? That seems to be a mutant/cultist he's operating on, and the man is not well restrained.
Well, all those things could be off-panel! We don't know!
And I'd always assumed the DarkMech had swapped most of their holy chants for "Fuck Mars, seriously, screw those guys, who needs them".
I wasn't referring to Hereteks in general, but Dark Mechanicus specifically. The Dark Mechanicus is limited to the rebel branch that was essentially dragged along with the Chaos Space Marines to keep all their gear running in the giant pit of unreality where they live (and boy, those guys must have it rough). Although I'm sure even the DarkMech has numerous different sects and sub-cults depending on what Legion they're most closely aligned with, Chaotic leaders they've fallen in with, and random acts of Warp-muckery that might have influenced their teachings.
I'm pretty sure hating Mars and the Loyalists who absurdly claim the Emperor is the Omnissiah is a requirement to join up with the Dark Mechanicus.
Of course, there are countless other "Hereteks" who would be ordinary Techpriests but for some political foible or minor dispute with Martian dogma - it's a pretty big galaxy - but most of those wouldn't be caught dead stitching up a Chaos cultist.
Ah, maybe from the perspective of the Adeptus Mechanicus, but then they have a vested interest in casting out and eliminating any group that might threaten their tech monopoly. So it's completely understandable that they'd label any Heretek with "Dark Mechanicus: shoot in face, thank you".
The Dark Mechanicus is very much a thing and identifies themselves as such. Aside from the Iron Warriors, which are very closely allied to them, they also function independently from the Chaos Space Marines that have to trade them protection, bodies and material for their help. Ultimately they're the ones who manage the limited technological resources within the Eye of Terror and keep the equipment working for the Chaos Space Marines, most of whom don't even realize that you can't fix every single machine with a wrench-axe and that one servo arm that Techmarines have mounted on their backpacks. They also control the Chaos Titan Legios, which is pretty much the only reason that they haven't been fully absorbed into the Legions as servants.