Doesn't "only" introduce the neccesary condition?

Hi Joyce, thanks for the question. You are right that "only" introduces a necessary condition. However, what we are presented with in this passage is actually "the only". This indicator is treated differently than "only" and it introduces a sufficient condition. That is why you are seeing a diagram that would appear to be backwards had we not taken this into account. I hope this helps. Please let me know if you have any other questions or would like me to elaborate further.