I do think he was speaking the truth here, atleast as far as he was aware: He could be killed because he had taken on a mortal form, and killing him would therefor kill Thancred. What he wasn't expecting was that we would still be able to gain a Blade of Light to use against him which allowed us to only kill him, and even then it was still not the permanent death that he also thought we couldn't do to Ascians later on.
Topic itself, it doesn't matter if the Ascians don't outright lie to us because they just about everything else they say is anywhere inbetween outright lies and absolute truths. Including letting people make wrong assumptions by not saying anything at all as was pointed out with Elidibus and Asahi.