He had also promised that if Loki didn't swear the oath, he would tell everyone that his hidden pathway was the Time Pathway and that he could attack anyone with a curse through their disciples.
But omitted those promises in his oaths now. It was intentional.
After missing that chunk of his promise, he waited for Loki to remember and say, "What about the other things you said you would tell people about me? Why didn't you swear not to tell now?"
And he would say, "Aha! I've got you. I knew that those things mattered to you."
It would be evidence that Loki had a Time Pathway and could curse people through their disciples. Unfortunately, Loki didn't take the bait.
It wasn't that he didn't want Wandering Thief to make those promises. If there was no risk of exposing himself, he might have insisted that Wandering Thief include the oath of confidentiality.
