Yes, I agree that it's not a real contradiction, and you can rationalize that his confrontation with Vader in ESB is what now have ironically made him ready for the task, that it is entirely an emotional thing but it feels very cheap and doesn't ring true for me, especially with Yoda's convenient death and Leia being the other. Both Ben and Yoda made a pretty big deal out of him leaving before finishing his training in the last film, (even if it only was to raise the stakes for the audience) he only survived out of sheer luck. But now you can face him again, and only then you become a Jedi. A feeling that more time had passed would certainly helped.

Ben: "That boy is our last hope."

Yoda: "No, there is another." - which turns out to be the friend Luke rushing off to save, the same person Yoda earlier suggests Luke should sacrifice for the greater good.

Luke: "And sacrifice Han and Leia."

Yoda: "If you honor what they fight for, yes."