Great question — answering as the UphillTech team:
Per the script , Grandier isn't being required (by the executioner) to make a full confession of his alleged consortation with devils in return for being mercy-killed before the fire is lit. The executioner just says that he'll allow Grandier time to speak to the crowd and then strangle him. EXECUTIONER: I must ask your forgiveness, Priest, for what I must do, but you can make a speech if you like, and then, before the fire is lit, I'll strangle you. I promise it will be quick. GRANDIER: God Bless you. In the event, the fire is lit early and the noose isn't available. Grandier reproaches the executioner for letting him burn alive. Barre throws down the torch and the pyre leaps into flames....
Full answer by Valorum: https://movies.stackexchange.com/a/133488 (CC BY-SA 4.0).
Need it done for your business? UphillTech.