There's a pause because hmm, does serving as the distraction while Amethyst and the angel are rescued count as betrayal? After all, Strange knows what Foster and the rest are doing and undoubtedly Nightshade would view them stealing two people from her realm as a betrayal. Still, she's outright considering this so at least in Strange's mind, that gives him a bit of an edge.

"Obviously, any actions that I have taken against your kind in the past I cannot undo. But as of this moment, I shall not partake in any further actions intentionally meant to betray you specifically. However, I think it's only fair that I ask the same of you: promise me that you will not partake in any actions intentionally meant to betray or harm me and I'll gladly give my oath."

That should cover his bases...he hopes.