[Oh, he doesn't doubt that the Doctor knows about Irminsul. Yet, he is not sure if the Doctor knows what it is capable of.]

Hm. I wonder if you still remember this conversation when you have returned to Teyvat.

[And then he smiles and cocks his head a little to the right.]

I am quite sure that the only thing you do know is that there has never been a Sixth Harbinger. That seat has been vacant for hundreds of years. [Then he takes a step closer.] But in case you haven't forgotten about my existence, feel free to come and punish me for being a traitor. Let them know...but...do not tell them how I did what I have done.

[Just for Lesser Lord Kusanali's sake. Not that he cares, but just in case...]