Well, once the interrogation started ( interrogations work in both directions ), the Mutilated had them, cants or no cants. The mere fact they took prisoners means:
1) They underestimated them, simple as that – as any prison guard underestimates the captured rendered feeble. Also, as Bakker says, "humility" was not their strong suit, so could be as prisoners they were unconcerned of any threat they posed and thought themselves protected with their cants, maybe the arrogance of sorcery towards the unmarked – magic/technology trumps intellect. Or immortality lends to immaturity - Bakker surely doesn't imbue much wisdom to his gods in Earwa; just a bunch of fuckers feeding on the fat/souls of humanity. And the non-men go insane with immortality. Could be they were so outclassed intellectually to the point they didn't understand the danger they put themselves in by bringing them into their mists.
2) They wanted to recruit them - I hadn't thought of this, but this has some merit, I think. Could be they wanted to see if they were damned likewise and enlist them to their common cause once the Dunyain were "educated" on hell. The war on Ishual was impressive, quite the campaign taking what appeared to me in the text massive losses ( a 1,000 sorcerers/magi/shriekers? ) and wanted some of that logos stuff on their side. Seems like you’all are leaning in this direction, appears I need to do a re-read.