Commentary Inf XXXIII 115-117

Dante's 'agreement' with Alberigo is utterly cynical; he has no intention of helping this sinner in any way, and says what he says only to get the sinner to disclose himself, swearing a misleading oath in order to reassure him (of course he is going to the 'bottom of the ice,' but not as a sinner for his eternal punishment, but as a very privileged visitor).