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I mean, saying you don’t owe loyalty to either the emperor or warmaster seems pretty heretical...

Yes?
I mean, they were renegade astartes. If they'd gone renegade during the great crusade they'd have been sanctioned, but during the horus heresy the Imperium had a bigger problem than the various blackshield groups (the clue's in the name) so they were largely ignored in the name of pragmatism until or unless they actively became a problem.