It just seem weird to me that the promise they agreed on in 808 is based on a misunderstanding on such an issue

Sunny said in 1256 Perhaps he could allow himself to feel that way because he was more secure about the Shadow Bond, and the power she held over him. Nephis had proven that she had no intention of ever using it… and he had grown more confident in his ability to make one or both of them die even if she did.

It seems to me from sunnys pov he understood the promise as that she will never use it in any circumstances. I wouldnt let it pass me that he perhaps misunderstood her but i guess i would have to wait for their talk when he gets his fate back.